# Many-Body Expanded Full Configuration Interaction. II. Strongly Correlated Regime

## TL;DR

This paper extends the MBE-FCI method to strongly correlated systems by introducing multideterminantal references, improving convergence and scalability for challenging chemical problems with high accuracy in extended basis sets.

## Contribution

It introduces multideterminantal expansion references into MBE-FCI, enhancing its ability to handle strong correlation and large basis sets with improved efficiency.

## Key findings

- Effective in bond dissociation tests for multiple molecules.
- Achieves near-ideal parallel scaling up to 25,000 processors.
- Demonstrates improved convergence with multideterminantal references.

## Abstract

In this second part of our series on the recently proposed many-body expanded full configuration interaction (MBE-FCI) method, we introduce the concept of multideterminantal expansion references. Through theoretical arguments and numerical validations, the use of this class of starting points is shown to result in a focussed compression of the MBE decomposition of the FCI energy, thus allowing chemical problems dominated by strong correlation to be addressed by the method. The general applicability and performance enhancements of MBE-FCI are verified for standard stress tests such as the bond dissociations in H$_2$O, N$_2$, C$_2$, and a linear H$_{10}$ chain. Furthermore, the benefits of employing a multideterminantal expansion reference in accelerating calculations of high accuracy are discussed, with an emphasis on calculations in extended basis sets. As an illustration of this latter quality of the MBE-FCI method, results for H$_2$O and C$_2$ in basis sets ranging from double- to pentuple-$\zeta$ quality are presented, demonstrating near-ideal parallel scaling on up to almost $25000$ processing units.