Localized Quantum Chemistry on Quantum Computers

TL;DR

This paper introduces LAS-UCC, a quantum algorithm that localizes multireference wave functions and combines QPE and UCCSD to efficiently compute ground state energies of large, strongly correlated chemical systems with improved scalability and accuracy.

Contribution

The paper presents LAS-UCC, a novel quantum algorithm that reduces resource requirements and improves accuracy for quantum chemistry calculations on quantum computers.

Findings

LAS-UCC scales linearly with system size for certain geometries.

LAS-UCC achieves higher accuracy than VQE with UCCSD and classical methods.

Demonstrated on dissociation of (H₂)₂ and breaking bonds in trans-butadiene.

Abstract

Quantum chemistry calculations of large, strongly correlated systems are typically limited by the computation cost that scales exponentially with the size of the system. Quantum algorithms, designed specifically for quantum computers, can alleviate this, but the resources required are still too large for today's quantum devices. Here we present a quantum algorithm that combines a localization of multireference wave functions of chemical systems with quantum phase estimation (QPE) and variational unitary coupled cluster singles and doubles (UCCSD) to compute their ground state energy. Our algorithm, termed "local active space unitary coupled cluster" (LAS-UCC), scales linearly with system size for certain geometries, providing a polynomial reduction in the total number of gates compared with QPE, while providing accuracy above that of the variational quantum eigensolver using the UCCSD ansatz and also above that of the classical local active space self-consistent field. The accuracy of LAS-UCC is demonstrated by dissociating (H$_2$)$_2$ into two H$_2$ molecules and by breaking the two double bonds in trans-butadiene and resources estimates are provided for linear chains of up to 20 H$_2$ molecules.