SDSPT2s: SDSPT2 with Selection

TL;DR

SDSPT2s introduces a selection and truncation approach to SDSPT2, enabling efficient and accurate treatment of large active spaces in multireference perturbation theory for complex molecular systems.

Contribution

It develops a selection and truncation scheme for SDSPT2, allowing application to larger systems with reduced computational cost while maintaining accuracy.

Findings

Effective reduction of reference space with minimal accuracy loss

Applicable to large, challenging molecular systems

Demonstrated success on several complex examples

Abstract

As an approximation to SDSCI [static-dynamic-static (SDS) configuration interaction (CI), a minimal MRCI; Theor. Chem. Acc. 133, 1481 (2014)], SDSPT2 [Mol. Phys. 115, 2696 (2017)] is a CI-like multireference (MR) second-order perturbation theory (PT2) that treats single and multiple roots on an equal footing. This feature permits the use of configuration selection over a large complete active space (CAS) $P$ to end up with a much reduced reference space $\tilde{P}$, which is connected only with a portion ($\tilde{Q}_1$) of the full first-order interacting space $Q$ connected to $P$. The effective interacting $\tilde{Q}$ space can further be truncated by an integral-based cutoff threshold. With marginal loss of accuracy, the selection-truncation procedure, along with an efficient evaluation and storage of internal contraction coefficients, renders SDSPT2s (SDSPT2 with selection) applicable to systems that cannot be handled by the parent CAS-based SDSPT2, as demonstrated by several challenging showcases.