VQE

This repository contains a code used to perform classical numerical simulations of molecules with the variational quantum eigensolver (VQE).The purpose of this repository is to be used as a benchmark to test and design ADAPT-VQE protocols (see Ref. 1).

• An implementation of the VQE is contained in src/vqe_runner.py
• The vqe_runner uses one of two different backends (in src/backends.py) to evaluate the expectation value of a quantum operator w.r.t. to a qubit state defined by an ansatz
• The ansatz is defined by a list of ansatz elements. Different types of ansatz elements are defined in src/ansatz_elements.py
• src/molecules/molecules.py contains a list of example molecular systems.

Adapt-VQE protocol (see Refs. 1,2,3,4):

• An implementation of the QEB-ADAPT-VQE protocol (Ref. 1) is contained in scripts/adapt_vqes/iqeb_vqe.py
• An implementation of the excited-QEB-ADAPT-VQE protocol (Ref. 3) is contained in scripts/adapt_vqes/excited_iqeb_vqe.py
• An implementation of the fermionic-ADAPT-VQE protocol (Ref. 2) is contained in scripts/adapt_vqes/adapt_vqe.py
• An implementation of the qubit-ADAPT-VQE protocol (Ref. 4) is contained in scripts/adapt_vqes/qubit_adapt_vqe.py

References:

1. https://www.nature.com/articles/s42005-021-00730-0
2. https://www.nature.com/articles/s41467-019-10988-2
3. https://arxiv.org/abs/2106.06296
4. https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.2.020310

Installation (Linux):

i. Install using the environment.yml file.

• note that this option may not work for some dependencies

ii. If using the environment.yml file does not work, install manually:

1. Create a new conda environment

2. Install the following libraries:

• qiskit 0.23.5 (or newer version)

• numpy 1.20.0 (or newer version)

• scipy 1.6.0

• ray

• pandas

  (Install the next three libraries in the given order. I recommend isntalling psi4 and openfermionpsi4 as given at https://github.com/quantumlib/OpenFermion-Psi4 )

• psi4

• openfermionpsi4 0.5 (or newer version)

• openfermion 1.0.0 (or newer version)

c) Make sure that the following directories are added to PYTHONPATH:

• "/home/ .. path/to .. /VQE"
• "/home/ .. path/to .. /VQE/src"

(To add these paths permanently you can use the following command: "conda develop '/home/ .. path/ to .. /VQE/'")