diff --git a/src/QMCDrivers/CorrelatedSampling/CSVMCUpdatePbyP.cpp b/src/QMCDrivers/CorrelatedSampling/CSVMCUpdatePbyP.cpp
index 906bea4a18..3e46903efb 100644
--- a/src/QMCDrivers/CorrelatedSampling/CSVMCUpdatePbyP.cpp
+++ b/src/QMCDrivers/CorrelatedSampling/CSVMCUpdatePbyP.cpp
@@ -73,7 +73,7 @@ void CSVMCUpdatePbyP::advanceWalker(Walker_t& thisWalker, bool recompute)
             stucked = false;
             ++nAccept;
             for (int ipsi = 0; ipsi < nPsi; ipsi++)
-              Psi1[ipsi]->acceptMove(W, iat);
+              Psi1[ipsi]->acceptMove(W, iat, true);
 
             W.acceptMove(iat, true);
             //Now we update ratioIJ.
diff --git a/src/QMCDrivers/DMC/DMCBatched.cpp b/src/QMCDrivers/DMC/DMCBatched.cpp
index 7cadb65ae8..250ec6f9a9 100644
--- a/src/QMCDrivers/DMC/DMCBatched.cpp
+++ b/src/QMCDrivers/DMC/DMCBatched.cpp
@@ -263,7 +263,7 @@ void DMCBatched::advanceWalkers(const StateForThread& sft,
         }
       }
 
-      TrialWaveFunction::flex_acceptMove(twf_accept_list, elec_accept_list, iat);
+      TrialWaveFunction::flex_acceptMove(twf_accept_list, elec_accept_list, iat, true);
       TrialWaveFunction::flex_rejectMove(twf_reject_list, iat);
 
       ParticleSet::flex_acceptMove(elec_accept_list, iat, true);
diff --git a/src/QMCDrivers/DMC/DMCUpdatePbyPFast.cpp b/src/QMCDrivers/DMC/DMCUpdatePbyPFast.cpp
index 378537a2be..c653189d99 100644
--- a/src/QMCDrivers/DMC/DMCUpdatePbyPFast.cpp
+++ b/src/QMCDrivers/DMC/DMCUpdatePbyPFast.cpp
@@ -108,7 +108,7 @@ void DMCUpdatePbyPWithRejectionFast::advanceWalker(Walker_t& thisWalker, bool re
         if (RandomGen() < prob)
         {
           ++nAcceptTemp;
-          Psi.acceptMove(W, iat);
+          Psi.acceptMove(W, iat, true);
           W.acceptMove(iat, true);
           rr_accepted += rr;
           gf_acc *= prob; //accumulate the ratio
diff --git a/src/QMCDrivers/RMC/RMCUpdatePbyP.cpp b/src/QMCDrivers/RMC/RMCUpdatePbyP.cpp
index accecd1e6e..4f26971c1e 100644
--- a/src/QMCDrivers/RMC/RMCUpdatePbyP.cpp
+++ b/src/QMCDrivers/RMC/RMCUpdatePbyP.cpp
@@ -179,7 +179,7 @@ void RMCUpdatePbyPWithDrift::advanceWalkersVMC()
         if (RandomGen() < prob)
         {
           ++nAcceptTemp;
-          Psi.acceptMove(W, iat);
+          Psi.acceptMove(W, iat, true);
           W.acceptMove(iat, true);
           rr_accepted += rr;
           gf_acc *= prob; //accumulate the ratio
@@ -310,7 +310,7 @@ void RMCUpdatePbyPWithDrift::advanceWalkersRMC()
         if (RandomGen() < prob)
         {
           ++nAcceptTemp;
-          Psi.acceptMove(W, iat);
+          Psi.acceptMove(W, iat, true);
           W.acceptMove(iat, true);
           rr_accepted += rr;
           gf_acc *= prob; //accumulate the ratio
diff --git a/src/QMCDrivers/VMC/SOVMCUpdatePbyP.cpp b/src/QMCDrivers/VMC/SOVMCUpdatePbyP.cpp
index 97a9fa8818..04fd977deb 100644
--- a/src/QMCDrivers/VMC/SOVMCUpdatePbyP.cpp
+++ b/src/QMCDrivers/VMC/SOVMCUpdatePbyP.cpp
@@ -111,7 +111,7 @@ void SOVMCUpdatePbyP::advanceWalker(Walker_t& thisWalker, bool recompute)
         {
           moved = true;
           ++nAccept;
-          Psi.acceptMove(W, iat);
+          Psi.acceptMove(W, iat, true);
           W.acceptMove(iat, true);
         }
         else
diff --git a/src/QMCDrivers/VMC/VMCBatched.cpp b/src/QMCDrivers/VMC/VMCBatched.cpp
index 095a1b8447..b71cae8193 100644
--- a/src/QMCDrivers/VMC/VMCBatched.cpp
+++ b/src/QMCDrivers/VMC/VMCBatched.cpp
@@ -190,7 +190,7 @@ void VMCBatched::advanceWalkers(const StateForThread& sft,
             elec_reject_list.push_back(crowd.get_walker_elecs()[i_accept]);
           }
 
-        TrialWaveFunction::flex_acceptMove(twf_accept_list, elec_accept_list, iat);
+        TrialWaveFunction::flex_acceptMove(twf_accept_list, elec_accept_list, iat, true);
         TrialWaveFunction::flex_rejectMove(twf_reject_list, iat);
 
         ParticleSet::flex_acceptMove(elec_accept_list, iat, true);
diff --git a/src/QMCDrivers/VMC/VMCUpdatePbyP.cpp b/src/QMCDrivers/VMC/VMCUpdatePbyP.cpp
index f39e1d7bdc..d9bbfb1c66 100644
--- a/src/QMCDrivers/VMC/VMCUpdatePbyP.cpp
+++ b/src/QMCDrivers/VMC/VMCUpdatePbyP.cpp
@@ -95,7 +95,7 @@ void VMCUpdatePbyP::advanceWalker(Walker_t& thisWalker, bool recompute)
         {
           moved = true;
           ++nAccept;
-          Psi.acceptMove(W, iat);
+          Psi.acceptMove(W, iat, true);
           W.acceptMove(iat, true);
         }
         else
diff --git a/src/QMCHamiltonians/NonLocalECPotential.cpp b/src/QMCHamiltonians/NonLocalECPotential.cpp
index 9683a7d800..f12d72fa75 100644
--- a/src/QMCHamiltonians/NonLocalECPotential.cpp
+++ b/src/QMCHamiltonians/NonLocalECPotential.cpp
@@ -351,7 +351,7 @@ int NonLocalECPotential::makeNonLocalMovesPbyP(ParticleSet& P)
       {
         GradType grad_iat;
         Psi.calcRatioGrad(P, iat, grad_iat);
-        Psi.acceptMove(P, iat);
+        Psi.acceptMove(P, iat, true);
         P.acceptMove(iat);
         NonLocalMoveAccepted++;
       }
@@ -371,7 +371,7 @@ int NonLocalECPotential::makeNonLocalMovesPbyP(ParticleSet& P)
           if (P.makeMoveAndCheck(iat, oneTMove->Delta))
           {
             Psi.calcRatioGrad(P, iat, grad_iat);
-            Psi.acceptMove(P, iat);
+            Psi.acceptMove(P, iat, true);
             P.acceptMove(iat);
             NonLocalMoveAccepted++;
           }
@@ -401,7 +401,7 @@ int NonLocalECPotential::makeNonLocalMovesPbyP(ParticleSet& P)
           if (P.makeMoveAndCheck(iat, oneTMove->Delta))
           {
             Psi.calcRatioGrad(P, iat, grad_iat);
-            Psi.acceptMove(P, iat);
+            Psi.acceptMove(P, iat, true);
             // mark all affected electrons
             markAffectedElecs(P.getDistTable(myTableIndex), iat);
             P.acceptMove(iat);
diff --git a/src/QMCWaveFunctions/AGPDeterminant.cpp b/src/QMCWaveFunctions/AGPDeterminant.cpp
index 15add95998..af6bffa35d 100644
--- a/src/QMCWaveFunctions/AGPDeterminant.cpp
+++ b/src/QMCWaveFunctions/AGPDeterminant.cpp
@@ -354,7 +354,7 @@ void AGPDeterminant::ratioDown(ParticleSet& P, int iat)
 
 /** move was accepted, update the real container
  */
-void AGPDeterminant::acceptMove(ParticleSet& P, int iat)
+void AGPDeterminant::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   LogValue += convertValueToLog(curRatio);
   //CurrentDet *= curRatio;
diff --git a/src/QMCWaveFunctions/AGPDeterminant.h b/src/QMCWaveFunctions/AGPDeterminant.h
index bfcb6f8d02..1f97924038 100644
--- a/src/QMCWaveFunctions/AGPDeterminant.h
+++ b/src/QMCWaveFunctions/AGPDeterminant.h
@@ -74,7 +74,7 @@ class AGPDeterminant : public WaveFunctionComponent
 
   /** move was accepted, update the real container
    */
-  void acceptMove(ParticleSet& P, int iat);
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
 
   /** move was rejected. copy the real container to the temporary to move on
    */
diff --git a/src/QMCWaveFunctions/ConstantOrbital.h b/src/QMCWaveFunctions/ConstantOrbital.h
index e8a3a8c042..d0b603b983 100644
--- a/src/QMCWaveFunctions/ConstantOrbital.h
+++ b/src/QMCWaveFunctions/ConstantOrbital.h
@@ -39,7 +39,7 @@ class ConstantOrbital : public WaveFunctionComponent
     return 0.0;
   }
 
-  virtual void acceptMove(ParticleSet& P, int iat) override {}
+  virtual void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false) override {}
 
   virtual void restore(int iat) override {}
 
diff --git a/src/QMCWaveFunctions/ExampleHeComponent.h b/src/QMCWaveFunctions/ExampleHeComponent.h
index 0a72118c65..cee185a695 100644
--- a/src/QMCWaveFunctions/ExampleHeComponent.h
+++ b/src/QMCWaveFunctions/ExampleHeComponent.h
@@ -48,7 +48,7 @@ class ExampleHeComponent : public WaveFunctionComponent
                            ParticleSet::ParticleGradient_t& G,
                            ParticleSet::ParticleLaplacian_t& L) override;
 
-  void acceptMove(ParticleSet& P, int iat) override {}
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false) override {}
 
   void restore(int iat) override {}
 
diff --git a/src/QMCWaveFunctions/FDLRWfn.cpp b/src/QMCWaveFunctions/FDLRWfn.cpp
index 001f6e5787..a8fbf04786 100644
--- a/src/QMCWaveFunctions/FDLRWfn.cpp
+++ b/src/QMCWaveFunctions/FDLRWfn.cpp
@@ -662,10 +662,10 @@ FDLRWfn::PsiValueType FDLRWfn::ratio(ParticleSet& P, int iat)
 /// \param[in]     iat      the id number of the moved particle
 ///
 ///////////////////////////////////////////////////////////////////////////////////////////////
-void FDLRWfn::acceptMove(ParticleSet& P, int iat)
+void FDLRWfn::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
-  m_wfn_xpd->acceptMove(P, iat);
-  m_wfn_xmd->acceptMove(P, iat);
+  m_wfn_xpd->acceptMove(P, iat, safe_to_delay);
+  m_wfn_xmd->acceptMove(P, iat, safe_to_delay);
 
   LogValue += convertValueToLog(curRatio);
 
diff --git a/src/QMCWaveFunctions/FDLRWfn.h b/src/QMCWaveFunctions/FDLRWfn.h
index ec12b0433b..fe2e054fb6 100644
--- a/src/QMCWaveFunctions/FDLRWfn.h
+++ b/src/QMCWaveFunctions/FDLRWfn.h
@@ -200,7 +200,7 @@ class FDLRWfn : public WaveFunctionComponent
 
   PsiValueType ratio(ParticleSet& P, int iat);
 
-  void acceptMove(ParticleSet& P, int iat);
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
 
   void restore(int iat);
 
diff --git a/src/QMCWaveFunctions/Fermion/DiracDeterminant.cpp b/src/QMCWaveFunctions/Fermion/DiracDeterminant.cpp
index 5a70bf13db..bced0b4aaa 100644
--- a/src/QMCWaveFunctions/Fermion/DiracDeterminant.cpp
+++ b/src/QMCWaveFunctions/Fermion/DiracDeterminant.cpp
@@ -223,12 +223,14 @@ void DiracDeterminant<DU_TYPE>::mw_ratioGrad(const std::vector<WaveFunctionCompo
 /** move was accepted, update the real container
 */
 template<typename DU_TYPE>
-void DiracDeterminant<DU_TYPE>::acceptMove(ParticleSet& P, int iat)
+void DiracDeterminant<DU_TYPE>::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   const int WorkingIndex = iat - FirstIndex;
   LogValue += convertValueToLog(curRatio);
   UpdateTimer.start();
   updateEng.acceptRow(psiM, WorkingIndex, psiV);
+  if (!safe_to_delay)
+    updateEng.updateInvMat(psiM);
   // invRow becomes invalid after accepting a move
   invRow_id = -1;
   if (UpdateMode == ORB_PBYP_PARTIAL)
diff --git a/src/QMCWaveFunctions/Fermion/DiracDeterminant.h b/src/QMCWaveFunctions/Fermion/DiracDeterminant.h
index bf898da378..7083b07be1 100644
--- a/src/QMCWaveFunctions/Fermion/DiracDeterminant.h
+++ b/src/QMCWaveFunctions/Fermion/DiracDeterminant.h
@@ -122,14 +122,14 @@ class DiracDeterminant : public DiracDeterminantBase
 
   /** move was accepted, update the real container
    */
-  void acceptMove(ParticleSet& P, int iat) override;
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false) override;
 
   void mw_acceptMove(const std::vector<WaveFunctionComponent*>& WFC_list,
                      const std::vector<ParticleSet*>& P_list,
-                     int iat) override
+                     int iat, bool safe_to_delay = false) override
   {
     for (int iw = 0; iw < WFC_list.size(); iw++)
-      WFC_list[iw]->acceptMove(*P_list[iw], iat);
+      WFC_list[iw]->acceptMove(*P_list[iw], iat, safe_to_delay);
   }
 
   void completeUpdates() override;
diff --git a/src/QMCWaveFunctions/Fermion/DiracDeterminantCUDA.h b/src/QMCWaveFunctions/Fermion/DiracDeterminantCUDA.h
index e9ce417844..bff5f04c25 100644
--- a/src/QMCWaveFunctions/Fermion/DiracDeterminantCUDA.h
+++ b/src/QMCWaveFunctions/Fermion/DiracDeterminantCUDA.h
@@ -157,7 +157,7 @@ class DiracDeterminantCUDA : public DiracDeterminantBase
     APP_ABORT("Calling DiracDeterminantCUDA::evaluateLog is illegal!");
   }
 
-  void acceptMove(ParticleSet& P, int iat) { APP_ABORT("Calling DiracDeterminantCUDA::acceptMove is illegal!"); }
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false) { APP_ABORT("Calling DiracDeterminantCUDA::acceptMove is illegal!"); }
 
   void restore(int iat) { APP_ABORT("Calling DiracDeterminantCUDA::restore is illegal!"); }
 
diff --git a/src/QMCWaveFunctions/Fermion/DiracDeterminantWithBackflow.cpp b/src/QMCWaveFunctions/Fermion/DiracDeterminantWithBackflow.cpp
index 3b3daf0857..b1aa8a5364 100644
--- a/src/QMCWaveFunctions/Fermion/DiracDeterminantWithBackflow.cpp
+++ b/src/QMCWaveFunctions/Fermion/DiracDeterminantWithBackflow.cpp
@@ -567,7 +567,7 @@ DiracDeterminantWithBackflow::LogValueType DiracDeterminantWithBackflow::evaluat
 
 /** move was accepted, update the real container
 */
-void DiracDeterminantWithBackflow::acceptMove(ParticleSet& P, int iat)
+void DiracDeterminantWithBackflow::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   LogValue += convertValueToLog(curRatio);
   UpdateTimer.start();
diff --git a/src/QMCWaveFunctions/Fermion/DiracDeterminantWithBackflow.h b/src/QMCWaveFunctions/Fermion/DiracDeterminantWithBackflow.h
index 4e4137dd5f..426dc8cf8e 100644
--- a/src/QMCWaveFunctions/Fermion/DiracDeterminantWithBackflow.h
+++ b/src/QMCWaveFunctions/Fermion/DiracDeterminantWithBackflow.h
@@ -123,7 +123,7 @@ class DiracDeterminantWithBackflow : public DiracDeterminantBase
 
   /** move was accepted, update the real container
    */
-  void acceptMove(ParticleSet& P, int iat);
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
 
   /** move was rejected. copy the real container to the temporary to move on
    */
diff --git a/src/QMCWaveFunctions/Fermion/MultiDiracDeterminant.cpp b/src/QMCWaveFunctions/Fermion/MultiDiracDeterminant.cpp
index a1eab14910..68bb6b5844 100644
--- a/src/QMCWaveFunctions/Fermion/MultiDiracDeterminant.cpp
+++ b/src/QMCWaveFunctions/Fermion/MultiDiracDeterminant.cpp
@@ -255,7 +255,7 @@ void MultiDiracDeterminant::copyFromBuffer(ParticleSet& P, WFBufferType& buf)
 
 /** move was accepted, update the real container
 */
-void MultiDiracDeterminant::acceptMove(ParticleSet& P, int iat)
+void MultiDiracDeterminant::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   WorkingIndex = iat - FirstIndex;
   switch (UpdateMode)
diff --git a/src/QMCWaveFunctions/Fermion/MultiDiracDeterminant.h b/src/QMCWaveFunctions/Fermion/MultiDiracDeterminant.h
index 612f47ed23..11aca2df67 100644
--- a/src/QMCWaveFunctions/Fermion/MultiDiracDeterminant.h
+++ b/src/QMCWaveFunctions/Fermion/MultiDiracDeterminant.h
@@ -155,7 +155,7 @@ class MultiDiracDeterminant : public WaveFunctionComponent
 
   /** move was accepted, update the real container
    */
-  void acceptMove(ParticleSet& P, int iat);
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
 
   /** move was rejected. copy the real container to the temporary to move on
    */
diff --git a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminant.cpp b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminant.cpp
index 1a8a678366..4bf7eabab0 100644
--- a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminant.cpp
+++ b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminant.cpp
@@ -393,7 +393,7 @@ WaveFunctionComponent::PsiValueType MultiSlaterDeterminant::ratio(ParticleSet& P
   }
 }
 
-void MultiSlaterDeterminant::acceptMove(ParticleSet& P, int iat)
+void MultiSlaterDeterminant::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   // this should depend on the type of update, ratio / ratioGrad
   // for now is incorrect fot ratio(P,iat,dG,dL) updates
diff --git a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminant.h b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminant.h
index f73efa7050..737eaa7a90 100644
--- a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminant.h
+++ b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminant.h
@@ -95,7 +95,7 @@ class MultiSlaterDeterminant : public WaveFunctionComponent
   virtual GradType evalGrad(ParticleSet& P, int iat);
   virtual PsiValueType ratioGrad(ParticleSet& P, int iat, GradType& grad_iat);
   virtual PsiValueType ratio(ParticleSet& P, int iat);
-  virtual void acceptMove(ParticleSet& P, int iat);
+  virtual void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
   virtual void restore(int iat);
 
   virtual void registerData(ParticleSet& P, WFBufferType& buf);
diff --git a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantFast.cpp b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantFast.cpp
index 85d5521e92..400ca22bb1 100644
--- a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantFast.cpp
+++ b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantFast.cpp
@@ -377,7 +377,7 @@ WaveFunctionComponent::PsiValueType MultiSlaterDeterminantFast::ratio(ParticleSe
   return curRatio;
 }
 
-void MultiSlaterDeterminantFast::acceptMove(ParticleSet& P, int iat)
+void MultiSlaterDeterminantFast::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   // this should depend on the type of update, ratio / ratioGrad
   // for now is incorrect fot ratio(P,iat,dG,dL) updates
@@ -390,7 +390,7 @@ void MultiSlaterDeterminantFast::acceptMove(ParticleSet& P, int iat)
   psiCurrent *= curRatio;
   curRatio = 1.0;
 
-  Dets[iat >= nels_up]->acceptMove(P, iat);
+  Dets[iat >= nels_up]->acceptMove(P, iat, safe_to_delay);
   //Dets[DetID[iat]]->acceptMove(P,iat);
 
   AccRejTimer.stop();
diff --git a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantFast.h b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantFast.h
index 566e93a5d5..e094e4bc0c 100644
--- a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantFast.h
+++ b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantFast.h
@@ -118,7 +118,7 @@ class MultiSlaterDeterminantFast : public WaveFunctionComponent
     APP_ABORT(" Need to implement MultiSlaterDeterminantFast::evaluateRatiosAlltoOne. \n");
   }
 
-  void acceptMove(ParticleSet& P, int iat) override;
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false) override;
   void restore(int iat) override;
 
   void registerData(ParticleSet& P, WFBufferType& buf) override;
diff --git a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantWithBackflow.cpp b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantWithBackflow.cpp
index 2a927c672c..70d96c3d84 100644
--- a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantWithBackflow.cpp
+++ b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantWithBackflow.cpp
@@ -355,7 +355,7 @@ WaveFunctionComponent::PsiValueType MultiSlaterDeterminantWithBackflow::ratio(Pa
   }
 }
 
-void MultiSlaterDeterminantWithBackflow::acceptMove(ParticleSet& P, int iat)
+void MultiSlaterDeterminantWithBackflow::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   // this should depend on the type of update, ratio / ratioGrad
   // for now is incorrect fot ratio(P,iat,dG,dL) updates
diff --git a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantWithBackflow.h b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantWithBackflow.h
index b866acf993..f7ee2d069d 100644
--- a/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantWithBackflow.h
+++ b/src/QMCWaveFunctions/Fermion/MultiSlaterDeterminantWithBackflow.h
@@ -65,7 +65,7 @@ class MultiSlaterDeterminantWithBackflow : public MultiSlaterDeterminant
   GradType evalGrad(ParticleSet& P, int iat);
   PsiValueType ratioGrad(ParticleSet& P, int iat, GradType& grad_iat);
   PsiValueType ratio(ParticleSet& P, int iat);
-  void acceptMove(ParticleSet& P, int iat);
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
   void restore(int iat);
 
   void registerData(ParticleSet& P, WFBufferType& buf);
diff --git a/src/QMCWaveFunctions/Fermion/SlaterDet.h b/src/QMCWaveFunctions/Fermion/SlaterDet.h
index 48e4f4e874..e82ad8095b 100644
--- a/src/QMCWaveFunctions/Fermion/SlaterDet.h
+++ b/src/QMCWaveFunctions/Fermion/SlaterDet.h
@@ -162,9 +162,9 @@ class SlaterDet : public WaveFunctionComponent
     Dets[det_id]->mw_restore(extract_Det_list(WFC_list, det_id), iat);
   }
 
-  virtual inline void acceptMove(ParticleSet& P, int iat) override
+  virtual inline void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false) override
   {
-    Dets[getDetID(iat)]->acceptMove(P, iat);
+    Dets[getDetID(iat)]->acceptMove(P, iat, safe_to_delay);
 
     LogValue = 0.0;
     for (int i = 0; i < Dets.size(); ++i)
@@ -173,7 +173,7 @@ class SlaterDet : public WaveFunctionComponent
 
   virtual void mw_acceptMove(const std::vector<WaveFunctionComponent*>& WFC_list,
                              const std::vector<ParticleSet*>& P_list,
-                             int iat) override
+                             int iat, bool safe_to_delay = false) override
   {
     constexpr RealType czero(0);
 
@@ -185,7 +185,7 @@ class SlaterDet : public WaveFunctionComponent
       const std::vector<WaveFunctionComponent*> Det_list(extract_Det_list(WFC_list, i));
 
       if (i == getDetID(iat))
-        Dets[i]->mw_acceptMove(Det_list, P_list, iat);
+        Dets[i]->mw_acceptMove(Det_list, P_list, iat, safe_to_delay);
 
       for (int iw = 0; iw < WFC_list.size(); iw++)
         WFC_list[iw]->LogValue += Det_list[iw]->LogValue;
diff --git a/src/QMCWaveFunctions/Fermion/SlaterDetWithBackflow.h b/src/QMCWaveFunctions/Fermion/SlaterDetWithBackflow.h
index cbd5189cbb..03c3c75f16 100644
--- a/src/QMCWaveFunctions/Fermion/SlaterDetWithBackflow.h
+++ b/src/QMCWaveFunctions/Fermion/SlaterDetWithBackflow.h
@@ -119,7 +119,7 @@ class SlaterDetWithBackflow : public SlaterDet
     return GradType();
   }
 
-  inline void acceptMove(ParticleSet& P, int iat)
+  inline void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false)
   {
     BFTrans->acceptMove(P, iat);
     for (int i = 0; i < Dets.size(); i++)
diff --git a/src/QMCWaveFunctions/Jastrow/CountingJastrow.h b/src/QMCWaveFunctions/Jastrow/CountingJastrow.h
index 4158dfe281..830e548ccc 100644
--- a/src/QMCWaveFunctions/Jastrow/CountingJastrow.h
+++ b/src/QMCWaveFunctions/Jastrow/CountingJastrow.h
@@ -380,7 +380,7 @@ class CountingJastrow : public WaveFunctionComponent
     return std::exp(static_cast<PsiValueType>(Jval_t - Jval));
   }
 
-  void acceptMove(ParticleSet& P, int iat)
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false)
   {
     C->acceptMove(P, iat);
     // update values for C, FC to those at proposed position
diff --git a/src/QMCWaveFunctions/Jastrow/J1OrbitalSoA.h b/src/QMCWaveFunctions/Jastrow/J1OrbitalSoA.h
index 403519dd22..d85e36f8a9 100644
--- a/src/QMCWaveFunctions/Jastrow/J1OrbitalSoA.h
+++ b/src/QMCWaveFunctions/Jastrow/J1OrbitalSoA.h
@@ -315,7 +315,7 @@ struct J1OrbitalSoA : public WaveFunctionComponent
   inline void restore(int iat) {}
 
   /** Accpted move. Update Vat[iat],Grad[iat] and Lap[iat] */
-  void acceptMove(ParticleSet& P, int iat)
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false)
   {
     if (UpdateMode == ORB_PBYP_RATIO)
     {
diff --git a/src/QMCWaveFunctions/Jastrow/J2OrbitalSoA.h b/src/QMCWaveFunctions/Jastrow/J2OrbitalSoA.h
index e3ae72e6b0..83d1170f81 100644
--- a/src/QMCWaveFunctions/Jastrow/J2OrbitalSoA.h
+++ b/src/QMCWaveFunctions/Jastrow/J2OrbitalSoA.h
@@ -262,7 +262,7 @@ class J2OrbitalSoA : public WaveFunctionComponent
 
   PsiValueType ratioGrad(ParticleSet& P, int iat, GradType& grad_iat);
 
-  void acceptMove(ParticleSet& P, int iat);
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
   inline void restore(int iat) {}
 
   /** compute G and L after the sweep
@@ -548,7 +548,7 @@ typename J2OrbitalSoA<FT>::PsiValueType J2OrbitalSoA<FT>::ratioGrad(ParticleSet&
 }
 
 template<typename FT>
-void J2OrbitalSoA<FT>::acceptMove(ParticleSet& P, int iat)
+void J2OrbitalSoA<FT>::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   // get the old u, du, d2u
   const auto& d_table = P.getDistTable(my_table_ID_);
diff --git a/src/QMCWaveFunctions/Jastrow/JeeIOrbitalSoA.h b/src/QMCWaveFunctions/Jastrow/JeeIOrbitalSoA.h
index 4afec1838d..0655886016 100644
--- a/src/QMCWaveFunctions/Jastrow/JeeIOrbitalSoA.h
+++ b/src/QMCWaveFunctions/Jastrow/JeeIOrbitalSoA.h
@@ -480,7 +480,7 @@ class JeeIOrbitalSoA : public WaveFunctionComponent
 
   inline void restore(int iat) {}
 
-  void acceptMove(ParticleSet& P, int iat)
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false)
   {
     const DistanceTableData& eI_table = P.getDistTable(ei_Table_ID_);
     const DistanceTableData& ee_table = P.getDistTable(ee_Table_ID_);
diff --git a/src/QMCWaveFunctions/Jastrow/OneBodyJastrowOrbital.h b/src/QMCWaveFunctions/Jastrow/OneBodyJastrowOrbital.h
index ce56de897e..ae54fe847f 100644
--- a/src/QMCWaveFunctions/Jastrow/OneBodyJastrowOrbital.h
+++ b/src/QMCWaveFunctions/Jastrow/OneBodyJastrowOrbital.h
@@ -386,7 +386,7 @@ class OneBodyJastrowOrbital : public WaveFunctionComponent
 
   inline void restore(int iat) {}
 
-  void acceptMove(ParticleSet& P, int iat)
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false)
   {
     LogValue += U[iat] - curVal;
     U[iat]   = curVal;
diff --git a/src/QMCWaveFunctions/Jastrow/OneBodySpinJastrowOrbital.h b/src/QMCWaveFunctions/Jastrow/OneBodySpinJastrowOrbital.h
index f94e979fd3..000a2d7e2f 100644
--- a/src/QMCWaveFunctions/Jastrow/OneBodySpinJastrowOrbital.h
+++ b/src/QMCWaveFunctions/Jastrow/OneBodySpinJastrowOrbital.h
@@ -384,7 +384,7 @@ class OneBodySpinJastrowOrbital : public WaveFunctionComponent
 
   inline void restore(int iat) {}
 
-  void acceptMove(ParticleSet& P, int iat)
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false)
   {
     U[iat]   = curVal;
     dU[iat]  = curGrad;
diff --git a/src/QMCWaveFunctions/Jastrow/RPAJastrow.cpp b/src/QMCWaveFunctions/Jastrow/RPAJastrow.cpp
index f6e98f6962..26dd01f431 100644
--- a/src/QMCWaveFunctions/Jastrow/RPAJastrow.cpp
+++ b/src/QMCWaveFunctions/Jastrow/RPAJastrow.cpp
@@ -280,10 +280,10 @@ RPAJastrow::PsiValueType RPAJastrow::ratioGrad(ParticleSet& P, int iat, GradType
 }
 
 
-void RPAJastrow::acceptMove(ParticleSet& P, int iat)
+void RPAJastrow::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   for (int i = 0; i < Psi.size(); i++)
-    Psi[i]->acceptMove(P, iat);
+    Psi[i]->acceptMove(P, iat, safe_to_delay);
 }
 
 void RPAJastrow::restore(int iat)
diff --git a/src/QMCWaveFunctions/Jastrow/RPAJastrow.h b/src/QMCWaveFunctions/Jastrow/RPAJastrow.h
index 669e012a66..85189075c7 100644
--- a/src/QMCWaveFunctions/Jastrow/RPAJastrow.h
+++ b/src/QMCWaveFunctions/Jastrow/RPAJastrow.h
@@ -80,7 +80,7 @@ struct RPAJastrow : public WaveFunctionComponent
   GradType evalGrad(ParticleSet& P, int iat);
   PsiValueType ratioGrad(ParticleSet& P, int iat, GradType& grad_iat);
 
-  void acceptMove(ParticleSet& P, int iat);
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
 
   void restore(int iat);
 
diff --git a/src/QMCWaveFunctions/Jastrow/TwoBodyJastrowOrbital.h b/src/QMCWaveFunctions/Jastrow/TwoBodyJastrowOrbital.h
index 7d59236b25..ed35d6320d 100644
--- a/src/QMCWaveFunctions/Jastrow/TwoBodyJastrowOrbital.h
+++ b/src/QMCWaveFunctions/Jastrow/TwoBodyJastrowOrbital.h
@@ -375,7 +375,7 @@ class TwoBodyJastrowOrbital : public WaveFunctionComponent
 
   inline void restore(int iat) {}
 
-  void acceptMove(ParticleSet& P, int iat)
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false)
   {
     DiffValSum += DiffVal;
     for (int jat = 0, ij = iat * N, ji = iat; jat < N; jat++, ij++, ji += N)
diff --git a/src/QMCWaveFunctions/Jastrow/eeI_JastrowOrbital.h b/src/QMCWaveFunctions/Jastrow/eeI_JastrowOrbital.h
index 8550435f25..db0ec6fc79 100644
--- a/src/QMCWaveFunctions/Jastrow/eeI_JastrowOrbital.h
+++ b/src/QMCWaveFunctions/Jastrow/eeI_JastrowOrbital.h
@@ -874,7 +874,7 @@ class eeI_JastrowOrbital : public WaveFunctionComponent
 
   inline void restore(int iat) {}
 
-  void acceptMove(ParticleSet& P, int iat)
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false)
   {
     const auto& eI_table = P.getDistTable(ei_table_index_);
     //      std::cerr << "acceptMove called.\n";
diff --git a/src/QMCWaveFunctions/Jastrow/kSpaceJastrow.cpp b/src/QMCWaveFunctions/Jastrow/kSpaceJastrow.cpp
index 4e10d94cbe..e086a80165 100644
--- a/src/QMCWaveFunctions/Jastrow/kSpaceJastrow.cpp
+++ b/src/QMCWaveFunctions/Jastrow/kSpaceJastrow.cpp
@@ -628,7 +628,7 @@ void kSpaceJastrow::restore(int iat)
   //if(NeedToRestore) Rhok -= delta_eikr;
 }
 
-void kSpaceJastrow::acceptMove(ParticleSet& P, int iat)
+void kSpaceJastrow::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   for (int i = 0; i < TwoBody_e2iGr_new.size(); i++)
     TwoBody_rhoG[i] += Delta_e2iGr(iat, i);
diff --git a/src/QMCWaveFunctions/Jastrow/kSpaceJastrow.h b/src/QMCWaveFunctions/Jastrow/kSpaceJastrow.h
index e9387dabe4..81c3ed8757 100644
--- a/src/QMCWaveFunctions/Jastrow/kSpaceJastrow.h
+++ b/src/QMCWaveFunctions/Jastrow/kSpaceJastrow.h
@@ -171,7 +171,7 @@ class kSpaceJastrow : public WaveFunctionComponent
   PsiValueType ratioGrad(ParticleSet& P, int iat, GradType& grad_iat);
 
   void restore(int iat);
-  void acceptMove(ParticleSet& P, int iat);
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
 
   // Allocate per-walker data in the PooledData buffer
   void registerData(ParticleSet& P, WFBufferType& buf);
diff --git a/src/QMCWaveFunctions/LatticeGaussianProduct.cpp b/src/QMCWaveFunctions/LatticeGaussianProduct.cpp
index 5a6ae71c78..1dd4070264 100644
--- a/src/QMCWaveFunctions/LatticeGaussianProduct.cpp
+++ b/src/QMCWaveFunctions/LatticeGaussianProduct.cpp
@@ -164,7 +164,7 @@ PsiValueType LatticeGaussianProduct::ratioGrad(ParticleSet& P, int iat, GradType
 
 void LatticeGaussianProduct::restore(int iat) {}
 
-void LatticeGaussianProduct::acceptMove(ParticleSet& P, int iat)
+void LatticeGaussianProduct::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   U[iat]   = curVal;
   dU[iat]  = curGrad;
diff --git a/src/QMCWaveFunctions/LatticeGaussianProduct.h b/src/QMCWaveFunctions/LatticeGaussianProduct.h
index 86aefe82cd..fe50feb3f2 100644
--- a/src/QMCWaveFunctions/LatticeGaussianProduct.h
+++ b/src/QMCWaveFunctions/LatticeGaussianProduct.h
@@ -68,7 +68,7 @@ struct LatticeGaussianProduct : public WaveFunctionComponent
 
   PsiValueType ratio(ParticleSet& P, int iat);
 
-  void acceptMove(ParticleSet& P, int iat);
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
 
   void restore(int iat);
 
diff --git a/src/QMCWaveFunctions/LinearOrbital.h b/src/QMCWaveFunctions/LinearOrbital.h
index 493b071abe..5e35d81ed6 100644
--- a/src/QMCWaveFunctions/LinearOrbital.h
+++ b/src/QMCWaveFunctions/LinearOrbital.h
@@ -58,7 +58,7 @@ class LinearOrbital : public WaveFunctionComponent
     return LogValue;
   }
 
-  virtual void acceptMove(ParticleSet& P, int iat) {}
+  virtual void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false) {}
 
   virtual void restore(int iat) {}
 
diff --git a/src/QMCWaveFunctions/TrialWaveFunction.cpp b/src/QMCWaveFunctions/TrialWaveFunction.cpp
index 11fb7e7a7c..1a3de67ba8 100644
--- a/src/QMCWaveFunctions/TrialWaveFunction.cpp
+++ b/src/QMCWaveFunctions/TrialWaveFunction.cpp
@@ -583,12 +583,12 @@ void TrialWaveFunction::flex_rejectMove(const RefVector<TrialWaveFunction>& wf_l
  * The proposed move of the iath particle is accepted.
  * All the temporary data should be incorporated so that the next move is valid.
  */
-void TrialWaveFunction::acceptMove(ParticleSet& P, int iat)
+void TrialWaveFunction::acceptMove(ParticleSet& P, int iat, bool safe_to_delay)
 {
   for (int i = 0, ii = ACCEPT_TIMER; i < Z.size(); i++, ii += TIMER_SKIP)
   {
     myTimers[ii]->start();
-    Z[i]->acceptMove(P, iat);
+    Z[i]->acceptMove(P, iat, safe_to_delay);
     myTimers[ii]->stop();
   }
   PhaseValue += PhaseDiff;
@@ -600,7 +600,7 @@ void TrialWaveFunction::acceptMove(ParticleSet& P, int iat)
 
 void TrialWaveFunction::flex_acceptMove(const RefVector<TrialWaveFunction>& wf_list,
                                         const RefVector<ParticleSet>& p_list,
-                                        int iat)
+                                        int iat, bool safe_to_delay)
 {
   if (wf_list.size() > 1)
   {
@@ -618,7 +618,7 @@ void TrialWaveFunction::flex_acceptMove(const RefVector<TrialWaveFunction>& wf_l
       ScopedTimer localtimer(wf_list[0].get().get_timers()[ii]);
       const auto wfc_list(extractWFCRefList(wf_list, i));
       wavefunction_components[i]->mw_acceptMove(convert_ref_to_ptr_list(wfc_list), convert_ref_to_ptr_list(p_list),
-                                                iat);
+                                                iat, safe_to_delay);
       for (int iw = 0; iw < wf_list.size(); iw++)
       {
         wf_list[iw].get().LogValue += std::real(wfc_list[iw].get().LogValue);
@@ -627,7 +627,7 @@ void TrialWaveFunction::flex_acceptMove(const RefVector<TrialWaveFunction>& wf_l
     }
   }
   else if (wf_list.size() == 1)
-    wf_list[0].get().acceptMove(p_list[0], iat);
+    wf_list[0].get().acceptMove(p_list[0], iat, safe_to_delay);
 }
 
 void TrialWaveFunction::completeUpdates()
diff --git a/src/QMCWaveFunctions/TrialWaveFunction.h b/src/QMCWaveFunctions/TrialWaveFunction.h
index 0ffd3bcf3a..50488d12a5 100644
--- a/src/QMCWaveFunctions/TrialWaveFunction.h
+++ b/src/QMCWaveFunctions/TrialWaveFunction.h
@@ -259,11 +259,11 @@ class TrialWaveFunction : public MPIObjectBase
   /* flexible batched version of rejectMove */
   static void flex_rejectMove(const RefVector<TrialWaveFunction>& wf_list, int iat);
 
-  void acceptMove(ParticleSet& P, int iat);
+  void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false);
   /* flexible batched version of acceptMove */
   static void flex_acceptMove(const RefVector<TrialWaveFunction>& wf_list,
                               const RefVector<ParticleSet>& p_list,
-                              int iat);
+                              int iat, bool safe_to_delay = false);
   void completeUpdates();
   /* flexible batched version of completeUpdates.  */
   void flex_completeUpdates(const std::vector<TrialWaveFunction*>& WF_list) const;
diff --git a/src/QMCWaveFunctions/WaveFunctionComponent.h b/src/QMCWaveFunctions/WaveFunctionComponent.h
index e969a30cc5..17f33c0a73 100644
--- a/src/QMCWaveFunctions/WaveFunctionComponent.h
+++ b/src/QMCWaveFunctions/WaveFunctionComponent.h
@@ -363,22 +363,24 @@ struct WaveFunctionComponent : public QMCTraits
   /** a move for iat-th particle is accepted. Update the current content.
    * @param P target ParticleSet
    * @param iat index of the particle whose new position was proposed
+   * @param safe_to_delay if true, delayed accept is safe.
    */
-  virtual void acceptMove(ParticleSet& P, int iat) = 0;
+  virtual void acceptMove(ParticleSet& P, int iat, bool safe_to_delay = false) = 0;
 
   /** moves of the iat-th particle on some walkers in a batch is accepted. Update the current content.
    *  Note that all the lists only include accepted walkers.
    * @param WFC_list the list of WaveFunctionComponent pointers of the same component in a walker batch
    * @param P_list the list of ParticleSet pointers in a walker batch
    * @param iat particle index
+   * @param safe_to_delay if true, delayed accept is safe.
    */
   virtual void mw_acceptMove(const std::vector<WaveFunctionComponent*>& WFC_list,
                              const std::vector<ParticleSet*>& P_list,
-                             int iat)
+                             int iat, bool safe_to_delay = false)
   {
 #pragma omp parallel for
     for (int iw = 0; iw < WFC_list.size(); iw++)
-      WFC_list[iw]->acceptMove(*P_list[iw], iat);
+      WFC_list[iw]->acceptMove(*P_list[iw], iat, safe_to_delay);
   }
 
   /** complete all the delayed updates, must be called after each substep or step during pbyp move
diff --git a/src/QMCWaveFunctions/tests/test_dirac_det.cpp b/src/QMCWaveFunctions/tests/test_dirac_det.cpp
index 1964371d5b..e8c6222af2 100644
--- a/src/QMCWaveFunctions/tests/test_dirac_det.cpp
+++ b/src/QMCWaveFunctions/tests/test_dirac_det.cpp
@@ -247,7 +247,6 @@ TEST_CASE("DiracDeterminant_first", "[wavefunction][fermion]")
   REQUIRE(det_ratio1 == ValueApprox(det_ratio));
 
   ddb.acceptMove(elec, 0);
-  ddb.completeUpdates();
 
   b(0, 0) = 3.455170657;
   b(0, 1) = -1.35124809;
@@ -345,7 +344,6 @@ TEST_CASE("DiracDeterminant_second", "[wavefunction][fermion]")
 
   ddb.acceptMove(elec, 0);
 
-
   PsiValueType det_ratio2 = ddb.ratioGrad(elec, 1, grad);
   LogValueType det_update2;
   simd::transpose(a_update2.data(), a_update2.rows(), a_update2.cols(), scratchT.data(), scratchT.rows(),
@@ -377,7 +375,6 @@ TEST_CASE("DiracDeterminant_second", "[wavefunction][fermion]")
   //check_value(det_ratio3, det_ratio3_val);
 
   ddb.acceptMove(elec, 2);
-  ddb.completeUpdates();
 
   simd::transpose(orig_a.data(), orig_a.rows(), orig_a.cols(), scratchT.data(), scratchT.rows(), scratchT.cols());
   dm.invert_transpose(scratchT, orig_a, det_update3);
@@ -474,10 +471,12 @@ TEST_CASE("DiracDeterminant_delayed_update", "[wavefunction][fermion]")
   REQUIRE(det_ratio1 == ValueApprox(det_ratio));
 
   // update of Ainv in ddc is delayed
-  ddc.acceptMove(elec, 0);
+  ddc.acceptMove(elec, 0, true);
   // force update Ainv in ddc using SM-1 code path
   ddc.completeUpdates();
 
+  check_matrix(a_update1, ddc.psiM);
+
   grad                    = ddc.evalGrad(elec, 1);
   PsiValueType det_ratio2 = ddc.ratioGrad(elec, 1, grad);
   simd::transpose(a_update2.data(), a_update2.rows(), a_update2.cols(), scratchT.data(), scratchT.rows(),
@@ -495,7 +494,7 @@ TEST_CASE("DiracDeterminant_delayed_update", "[wavefunction][fermion]")
   REQUIRE(det_ratio2 == ValueApprox(det_ratio2_val));
 
   // update of Ainv in ddc is delayed
-  ddc.acceptMove(elec, 1);
+  ddc.acceptMove(elec, 1, true);
 
   grad                    = ddc.evalGrad(elec, 2);
   PsiValueType det_ratio3 = ddc.ratioGrad(elec, 2, grad);
@@ -514,7 +513,7 @@ TEST_CASE("DiracDeterminant_delayed_update", "[wavefunction][fermion]")
   //check_value(det_ratio3, det_ratio3_val);
 
   // maximal delay reached and Ainv is updated fully
-  ddc.acceptMove(elec, 2);
+  ddc.acceptMove(elec, 2, true);
   ddc.completeUpdates();
 
   // fresh invert orig_a