Doubling the size of quantum selected configuration interaction based on seniority-zero space and its application to QC-QSCI-AFQMC

TL;DR

This paper introduces DOCI-QSCI, a method that doubles the accessible orbital space for quantum configuration interaction, and combines it with ph-AFQMC to accurately model complex molecular systems.

Contribution

The paper presents a novel approach combining seniority-zero space sampling with a Cartesian product expansion and ph-AFQMC, enhancing the accuracy and size of quantum simulations.

Findings

DOCI-QSCI achieves accuracy comparable to complete-active-space methods.

The method handles larger active spaces than traditional QSCI.

It outperforms single-reference CCSD(T) in tested cases.

Abstract

We propose doubly occupied configuration interaction-quantum selected configuration interaction (DOCI-QSCI), which samples from the seniority-zero space. While the use of this space effectively doubles the qubit budget, equaling the number of spatial orbitals, this sector restriction can compromise quantitative accuracy. To compensate for this, we expand sampled bitstrings via their Cartesian product into a larger space that includes seniority-breaking determinants. The resulting wave function is also proposed using the trial state in phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) to recover dynamical correlations across the full orbital space (DOCI-QSCI-AFQMC). We evaluate the proposed methods on the H6 chain, N2 dissociation, and the addition of singlet O2 to a BODIPY dye. For the H6 chain, DOCI-QSCI-AFQMC reproduces the accuracy of the level of the complete-active-space counterpart with the quantum device ibm kobe. For N2 and BODIPY-O2, with (14e, 28o) and up to (20e, 20o) active spaces, it yields reasonable results, whereas single-reference CCSD(T) fails qualitatively. These results demonstrate that the DOCI-QSCI doubles the orbital space accessible to conventional QSCI and subsequent ph-AFQMC post-processing delivers reasonably high accuracy.