Chemistry

image.png

import pyscf
import pyscf.cc
import pyscf.mcscf
import ffsim
import numpy as np
import matplotlib.pyplot as plt
 
from qiskit import QuantumCircuit, QuantumRegister
from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager
 
from qiskit_ibm_runtime import QiskitRuntimeService
from qiskit_ibm_runtime import SamplerV2 as Sampler

# Specify molecule properties
open_shell = False
spin_sq = 0
 
# Build N2 molecule
mol = pyscf.gto.Mole()
mol.build(
    atom=[["N", (0, 0, 0)], ["N", (1.0, 0, 0)]],
    basis="cc-pvdz",
    symmetry="Dooh",
)
 
# Define active space
n_frozen = 2
active_space = range(n_frozen, mol.nao_nr())
 
# Get molecular integrals
scf = pyscf.scf.RHF(mol).run()
num_orbitals = len(active_space)
n_electrons = int(sum(scf.mo_occ[active_space]))
num_elec_a = (n_electrons + mol.spin) // 2
num_elec_b = (n_electrons - mol.spin) // 2
cas = pyscf.mcscf.CASCI(scf, num_orbitals, (num_elec_a, num_elec_b))
mo = cas.sort_mo(active_space, base=0)
hcore, nuclear_repulsion_energy = cas.get_h1cas(mo)
eri = pyscf.ao2mo.restore(1, cas.get_h2cas(mo), num_orbitals)
 
# Store reference energy from SCI calculation performed separately
exact_energy = -109.22690201485733

converged SCF energy = -108.929838385609

# Get CCSD t2 amplitudes for initializing the ansatz
ccsd = pyscf.cc.CCSD(
    scf, frozen=[i for i in range(mol.nao_nr()) if i not in active_space]
).run()
t1 = ccsd.t1
t2 = ccsd.t2

E(CCSD) = -109.2177884185544  E_corr = -0.2879500329450041

n_reps = 1
alpha_alpha_indices = [(p, p + 1) for p in range(num_orbitals - 1)]
alpha_beta_indices = [(p, p) for p in range(0, num_orbitals, 4)]
 
ucj_op = ffsim.UCJOpSpinBalanced.from_t_amplitudes(
    t2=t2,
    t1=t1,
    n_reps=n_reps,
    interaction_pairs=(alpha_alpha_indices, alpha_beta_indices),
)
 
nelec = (num_elec_a, num_elec_b)
 
# create an empty quantum circuit
qubits = QuantumRegister(2 * num_orbitals, name="q")
circuit = QuantumCircuit(qubits)
 
# prepare Hartree-Fock state as the reference state and append it to the quantum circuit
circuit.append(ffsim.qiskit.PrepareHartreeFockJW(num_orbitals, nelec), qubits)
 
# apply the UCJ operator to the reference state
circuit.append(ffsim.qiskit.UCJOpSpinBalancedJW(ucj_op), qubits)
circuit.measure_all()

circuit.decompose(reps=3).draw()

#TODO support more customized gates such like sdg, s, square root x

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