h-chain example

"""
pip install openfermion pyscf openfermionpyscf fqe
"""
import openfermion as of
from openfermionpyscf import run_pyscf
import fqe
from fqe.openfermion_utils import integrals_to_fqe_restricted


def main():
    num_atoms = 2
    atom_type = 'H'
    bond_distance = 2.4
    molecule = of.chem.chemical_series.make_atomic_lattice(nx_atoms=1,
                                                           ny_atoms=num_atoms,
                                                           nz_atoms=1,
                                                           basis='sto-3g',
                                                           atom_type=atom_type,
                                                           spacing=bond_distance
                                                           )
    print(molecule)
    molecule = run_pyscf(molecule, run_scf=True, run_fci=True)
    n_electrons = molecule.n_electrons
    sz = 0
    norbs = molecule.n_orbitals

    oei, tei = molecule.get_integrals()
    print(oei)
    fermion_hamiltonian = of.get_fermion_operator(molecule.get_molecular_hamiltonian())
    fqe_ham = integrals_to_fqe_restricted(oei, tei)

    print(of.jordan_wigner(fermion_hamiltonian))

    # etc, etc, etc,

    hf_wf = fqe.Wavefunction([[n_electrons, sz, norbs]])
    hf_wf.set_wfn(strategy='hartree-fock')
    hf_wf.print_wfn()
    print(hf_wf.expectationValue(fqe_ham).real + molecule.nuclear_repulsion,
          molecule.hf_energy)  # these should be the same


if __name__ == "__main__":
    main()