run_pyscf does not work on PyscfMolecularData

[kevinsung]

Input:

from openfermionpyscf import PyscfMolecularData, run_pyscf

geometry = [('Li', (0., 0., 0.)), ('H', (0., 0., 1.4))]
basis = 'sto-3g'
multiplicity = 1
charge = 0

molecule = run_pyscf(
        PyscfMolecularData(geometry, basis, multiplicity, charge)
)

Output:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-1-bf13348427ff> in <module>()
      7 
      8 molecule = run_pyscf(
----> 9         PyscfMolecularData(geometry, basis, multiplicity, charge)
     10 )

~/Projects/OpenFermion-PySCF/openfermionpyscf/_run_pyscf.py in run_pyscf(molecule, run_scf, run_mp2, run_cisd, run_ccsd, run_fci, verbose)
    143 
    144     # Populate fields.
--> 145     molecule.canonical_orbitals = pyscf_scf.mo_coeff.astype(float)
    146     molecule.orbital_energies = pyscf_scf.mo_energy.astype(float)
    147 

AttributeError: can't set attribute

The reason is that PyscfMolecularData has canonical_orbitals, etc., as properties rather than attributes. One could fix this by adding setters, but I don't know what the correct design or intended usage is. If we can calculate stuff on the fly, then perhaps we don't even need to perform run_pyscf on a PyscfMolecularData? @sunqm

[sunqm]

I like the design to compute orbitals and other quantities on the fly. At this stage, the overhead to recompute things is negligible. By the time when the interface was implemented, molecule.canonical_orbitals was a regular attribute than the property. To make the interface compatible with the old implementation, run_pyscf is kept and attributes like molecule.canonical_orbitals, molecule.orbital_energies are initialized in run_pyscf.

When computing things on the fly, the symmetry of the system needs to be carefully considered. The orbitals may be differed by a phase from run to run. If orbitals are recomputed, the MO integrals may be varied during the simulation accordingly, which may cause numerical instability.