quantum_chemistry.py

'''
This program uses some methods from quantum chemistry 
(and doesn't produce anything good yet)
'''

import numpy as np
from matplotlib.pyplot import plot, draw, show
import pylab, os
from qiskit import *
#from qiskit.chemistry import QiskitChemistry # does no longer exist
from qiskit_nature import *

os.environ["QT_QPA_PLATFORM"] = "xcb"

# input dictionary to configure Qiskit Chemistry for the chemistry problem
qiskit_chemistry_dict = {
    'problem': {'random_seed': 750},
    'driver' : {'name': 'PYSCF'},
    'PYSCF': {'atom': '', 'basis' : 'sto3g'},
    'operator': {'name': 'hamiltonian', 
                'transformation': 'full',
                'qubit_mapping': 'parity',
                'two_qubit_reduction': True},
    'algorithm': {},
}

molecule = 'H .0 .0 -{0}; H .0 .0 {0}'
algorithms = [{'name': 'VQE','operator_mode': 'paulis'},
                {'name': 'ExactEigensolver'}
            ]
optimizer = {'name': 'SPSA', 'max_trials': 200}
variational_form = {'name': 'RYRZ', 'depth': 3, 'entanglement': 'full'}
backend = {'provider': 'qiskit.BasicAer',
            'name': 'qasm_simulator',
             'shots': 1024}
start = 0.5 # start distance
increment = 0.5 # how much to increase distance by
steps = 20 # number of steps to increase by
energies = np.empty([len(algorithms), steps+1])
hf_energies = np.empty(steps+1)
distances = np.empty(steps+1)
print('Processing step __', end='')
for i in range(steps+1):
    print('\b\b{:2d}'.format(i), end='',flush=True)
    d = start + i*increment/steps
    qiskit_chemistry_dict['PYSCF']['atom'] = molecule.format(d/2)

    for j in range(len(algorithms)):
        qiskit_chemistry_dict['algorithm'] = algorithms[j]
        if algorithms[j]['name'] == 'VQE':
            qiskit_chemistry_dict['optimizer'] = optimizer
            qiskit_chemistry_dict['variational_form'] = variational_form
            qiskit_chemistry_dict['backend'] = backend
        else:
            # This does no longer work as qiskit.chemistry is deprecated
            # The functions may be contained in qiskit_nature
            '''
            qiskit_chemistry_dict.pop('optimizer')
            qiskit_chemistry_dict.pop('variational_form')
            qiskit_chemistry_dict.pop('backend')
            solver = QiskitChemistry()
            result = solver.run(qiskit_chemistry_dict)
            energies[j][i] = result['energy']
            hf_energies[i] = result['hf_energy']
            distances[i] = d
            '''
        print(' --- complete')
        print('Distances: ', distances)
        print('Energies: ', energies)
        print('Hartree-Fock energies: ', hf_energies)
pylab.plot(distances, hf_energies, label='Hartree-Fock')
for j in range(len(algorithms)):
    pylab.plot(distances, energies[j], label=algorithms[j])
    pylab.xlabel('Interatomic distance (Angstrom)')
    pylab.ylabel('Energy (Hartree)')
    pylab.title('H2 Ground State Energy')
    pylab.legend(loc='upper right')
    draw()
    show(block=True)        

    pylab.plot(distances, np.subtract(hf_energies, energies[1]), label='Hartree-Fock')
    pylab.plot(distances, np.subtract(energies[0], energies[1]), label=algorithms[0])
    pylab.xlabel('Interatomic distance (Angstrom)')
    pylab.ylabel('Energy (Hartree)')
    pylab.title('Energy difference from ExactEigensolver')
    pylab.legend(loc='upper left')
    draw()
    show(block=True)