Update dependencies in quantum chemistry tutorial

demonstrations/tutorial_quantum_chemistry.py

@@ -40,10 +40,7 @@
 the structure of the molecule in terms of the symbols and the coordinates of
 the atoms. Next, we describe how to solve the `Hartree-Fock
 equations <https://en.wikipedia.org/wiki/Hartree%E2%80%93Fock_method>`_ for the target
-molecule by interfacing with classical quantum chemistry packages. Then, we
-outline how to decompose the fermionic Hamiltonian into a set of Pauli operators
-whose expectation values can be measured in a quantum computer to estimate the total energy
-of the molecule. Finally, we discuss some advanced features that can be used to simulate
+molecule. Finally, we discuss some advanced features that can be used to simulate
 more complicated systems.
 
 Let's get started!
@@ -70,7 +67,7 @@
 coordinates = np.array([-0.0399, -0.0038, 0.0, 1.5780, 0.8540, 0.0, 2.7909, -0.5159, 0.0])
 
 ##############################################################################
-# The :func:`~.pennylane_qchem.qchem.read_structure` function can also be used to read the
+# The :func:`~.pennylane.qchem.read_structure` function can also be used to read the
 # molecular geometry from an external file.
 
 
@@ -79,9 +76,7 @@
 symbols, coordinates = qchem.read_structure("h2o.xyz")
 
 ##############################################################################
-# The xyz format is supported out of the box. If
-# `Open Babel <http://openbabel.org/wiki/Main_Page>`_ is installed, any
-# format recognized by Open Babel is also supported by PennyLane.
+# The xyz format is supported.
 #
 # Solving the Hartree-Fock equations
 # ----------------------------------
@@ -101,17 +96,8 @@
 # field generated by all other electrons [#pople1977]_. The optimized coefficients are precisely
 # what we need to build the second-quantized Hamiltonian.
 #
-# We can call the function :func:`~.pennylane_qchem.qchem.meanfield` to perform
-# the Hartree-Fock calculation using either the quantum chemistry package `PySCF
-# <https://sunqm.github.io/pyscf/>`_, or `Psi4 <http://www.psicode.org/>`_. Here
-# we use PySCF, which is the default option.
-
-hf_file = qchem.meanfield(symbols, coordinates)
-
-##############################################################################
-# This function creates a local file that will later be used to compute the Hamiltonian.
-#
-# .. _hamiltonian:
+# PennyLane provides a differentiable Hartree-Fock solver and the functionality to construct a
+# fully-differentiable molecular Hamiltonian.
 #
 # Building the Hamiltonian
 # ------------------------
@@ -149,20 +135,9 @@
 #
 # where :math:`C_j` is a scalar coefficient and :math:`\sigma_i` represents an
 # element of the Pauli group :math:`\{ I, X, Y, Z \}`.
-#
-# This fermionic-to-qubit transformation is done using
-# the :func:`~.pennylane_qchem.qchem.decompose` function, which
-# uses `OpenFermion <https://github.com/quantumlib/OpenFermion>`_
-# to compute the electron integrals using the previously generated
-# results of the mean field calculation. Then, it builds the fermionic Hamiltonian and
-# maps it to the qubit representation.
-
-qubit_hamiltonian = qchem.decompose(hf_file, mapping="jordan_wigner")
-print("Qubit Hamiltonian of the water molecule")
-print(qubit_hamiltonian)
 
 ##############################################################################
-# It is often convenient to use the :func:`~.pennylane_qchem.qchem.molecular_hamiltonian`
+# It PennyLane we have the :func:`~.pennylane.qchem.molecular_hamiltonian`
 # function which encapsulates all the steps explained above. It simplifies the process of building
 # the electronic Hamiltonian to a single line of code. We just need to input the
 # symbols and the nuclear coordinates of the molecule, as shown below:
@@ -173,15 +148,10 @@
 print(H)
 
 ##############################################################################
-# Additionally, if you have built your electronic Hamiltonian independently using
-# `OpenFermion <https://github.com/quantumlib/OpenFermion>`_ tools, it can
-# be readily converted to a PennyLane observable using the
-# :func:`~.pennylane_qchem.qchem.convert_observable` function.
-#
 # Advanced features
 # -----------------
-# The :func:`~.pennylane_qchem.qchem.meanfield` function allows us to define additional keyword
-# arguments to solve the Hartree-Fock equations of more complicated systems.
+# The :func:`~.pennylane.qchem.molecular_hamiltonian` function allows us to define additional
+# keyword arguments to solve the Hartree-Fock equations of more complicated systems.
 # The net charge of the molecule may be specified to simulate positively or negatively
 # charged molecules. For a neutral system we choose
 
@@ -242,7 +212,7 @@
 # For the water molecule in a minimal basis set we have a total of ten electrons
 # and seven molecular orbitals. In this example we define an symmetric active space with
 # four electrons and four active orbitals using
-# the :func:`~.pennylane_qchem.qchem.active_space` function:
+# the :func:`~.pennylane.qchem.active_space` function:
 
 electrons = 10
 orbitals = 7
@@ -256,7 +226,7 @@
 print("Number of qubits: {:}".format(2 * len(active)))
 
 ##############################################################################
-# Finally, we use the :func:`~.pennylane_qchem.qchem.molecular_hamiltonian` function to
+# Finally, we use the :func:`~.pennylane.qchem.molecular_hamiltonian` function to
 # build the resulting Hamiltonian of the water molecule:
 
 H, qubits = qchem.molecular_hamiltonian(
@@ -278,6 +248,32 @@
 # observable is an approximation of the Hamiltonian generated in the
 # section :ref:`hamiltonian`.
 #
+# OpenFermion-PySCF backend
+# -------------------------
+# The :func:`~.pennylane.qchem.molecular_hamiltonian` function can be also used to construct the
+# molecular Hamiltonian with a non-differentiable backend that uses the
+# `OpenFermion-PySCF <https://github.com/quantumlib/OpenFermion-PySCF>`_ plugin interfaced with the
+# electronic structure package `PySCF <https://github.com/sunqm/pyscf>`_. The non-differentiable
+# backend can be selected by setting `method='pyscf'` in
+# :func:`~.pennylane.qchem.molecular_hamiltonian`:
+
+symbols = ["H", "H"]
+geometry = np.array([[0.0, 0.0, 0.0], [0.0, 0.0, 2.0]])
+H, qubits = qchem.molecular_hamiltonian(symbols, geometry, method='pyscf')
+
+##############################################################################
+# The non-differentiable backend requires the ``OpenFermion-PySCF`` plugin to be installed by the
+# user with
+#
+# .. code-block:: bash
+#
+# pip install openfermionpyscf
+#
+# Additionally, if you have built your electronic Hamiltonian independently using
+# `OpenFermion <https://github.com/quantumlib/OpenFermion>`_ tools, it can
+# be readily converted to a PennyLane observable using the
+# :func:`~.pennylane.qchem.import_operator` function.
+#
 # You have completed the tutorial! Now, select your favorite molecule and build its electronic
 # Hamiltonian.
 # To see how simple it is to implement the VQE algorithm to compute the ground-state energy of