Matrix product states with large sites

TL;DR

This paper introduces a specialized matrix product state (MPS) structure with large sites, optimized for multireference theories, and demonstrates its effectiveness through applications to complex molecular systems.

Contribution

It proposes a novel MPS cluster structure focusing on the first and last orbitals, enabling efficient multireference calculations with large sites.

Findings

Efficient implementation of multireference methods using large-site MPS.

Successful application to complex molecules like nitrogen and chromium dimers.

Analysis of multireference approximation quality.

Abstract

We explore various ways to group orbitals into clusters in a matrix product state (MPS). We explain how a generic cluster MPS can often lead to an increase in computational cost and instead propose a special cluster structure, involving only the first and last orbitals/sites, with a wider scope for computational advantage. This structure is a natural formalism to describe correlated multireference (MR) theories. We demonstrate the flexibility and usefulness of this approach by implementing various uncontracted MR configuration interaction, perturbation and linearized coupled cluster theories using an MPS with large cluster sites. Applications to the nitrogen dimer, the chromium dimer, and benzene, including up to triple excitations in the external space, demonstrate the utility of an MPS with up to two large sites. We use our results to analyze the quality of different multireference approximations.