Multi-Reference UCCSD Variational Quantum Algorithm for Molecular Ground State Energies

TL;DR

This paper introduces a quantum algorithm that efficiently computes molecular ground state energies with high precision, using a multi-reference approach that reduces quantum resource requirements and maintains accuracy across bond lengths.

Contribution

The paper presents a novel MR-UCCSD quantum algorithm that conserves particle number, simplifies computation, and achieves high-precision results with fewer quantum gates.

Findings

Achieves errors below 10^{-5} Hartree across bond lengths.

Uses only hundreds of CNOT gates for high-precision calculations.

Maintains accuracy comparable to single-reference UCCSD.

Abstract

We implement the Multi-Reference Unitary Coupled Cluster Singles and Doubles (MR-UCCSD) model with a quantum circuit that conserves the particle number to study the ground state energies of LiH, BeH$_2$, and H$_6$. This approach simplifies the MR-UCCSD computation by integrating quantum computing techniques, and reduces its complexity. As a profit of the better MR states, our MR-UCCSD approach satisfies systematically the predefined errors below 10$^{-5}$ Hartree,which is the highest precision of single reference UCCSD approach, along the whole bond length with only hundreds of CNOT gates, and meets satisfactory the requirements of both computational precision and quantum resource reduction.