Multideterminant code fails with only alpha determinants (no down electrons)

[anbenali]

Describe the bug
When beta determinants are absent / only up electrons, the multideterminant code is broken. QMCPACK will request removing particle set for beta electrons with size 0, but multideterminant will require and sposet down. An sposet down with size 0 will force qmcpack to exit asking to remove particleset with 0 particle.

To Reproduce
Steps to reproduce the behavior:

1. download example files from link: https://anl.box.com/s/p3xn8r64rpk8h0zj3nredxjyqs97e81o
2. run:

qmcpack Li-quadruplet-MD.qmc.in-wfj.xml

Expected behavior
Code will fail if particle set with down electron specified with size 0;

Fatal Error. Aborting at ParticleSet::resetGroups() Failed. ParticleSet 'e' has group 'd' containing 0 particles. Remove this group from input!

When commented out from Li-quadruplet-MD.structure.xml

Norm of ci vector (sum of ci^2): 1
Found 40 unique up determinants.
Found 1 unique down determinants.
Number of terms in pairs array: 39
Fatal Error. Aborting at Error in SlaterDetBuilder::createMSDFast, problems with ci configuration list.
Abort(1) on node 0 (rank 0 in comm 0): application called MPI_Abort(MPI_COMM_WORLD, 1) - process 0

System:
all

[camelto2]

Also relevant for molecular spin-orbit calculations, as there is only electron group by construction

[camelto2]

For reference, I have a hacked version of qmcpack that works for this. Essentially, every variable that is hardcoded as X_up and X_dn I have generalized to be a vector called Xs and depends on the number of groups in the electron particleset. Since this part of the code is being redesigned in the future, in case anyone needs this working for a particular problem feel free to contact me for the hacked code

[ye-luo]

VMC runs fine after removing beta

    <determinantset type="MolecularOrbital" name="LCAOBSet" source="ion0" transform="yes" cuspCorrection="yes" href="Li-quadruplet-MD.h5">
      <sposet basisset="LCAOBSet" name="spo-up" size="18" cuspInfo="spo-up.cuspInfo.xml">
        <occupation mode="ground"/>
        <coefficient size="18" spindataset="0"/>
      </sposet>
      <multideterminant optimize="no" spo_0="spo-up">
        <detlist size="40" type="DETS" nc0="0" ne0="3" nstates="18" cutoff="1e-20" href="Li-quadruplet-MD.h5"/>
      </multideterminant>
    </determinantset>