The Shape of Full Configuration Interaction to Come

TL;DR

This paper discusses the future of full configuration interaction (FCI) theory, reviewing its history, current approximations, and challenges, emphasizing the need for robustness and applicability to larger, more complex systems.

Contribution

It provides a conceptual perspective on FCI's evolution, compares key approximations using recent benchmarks, and highlights future challenges for extending its applicability.

Findings

FCI remains essential for certain applications.

Modern approximations need improved robustness and efficiency.

Progress has been significant, but challenges in scaling persist.

Abstract

We present a perspective on what the future holds for full configuration interaction (FCI) theory, with an emphasis on conceptual rather than technical details. Upon revisiting the early history of FCI, a number of its key contemporary approximations are compared on as equal a footing as possible, using a recent blind challenge on the benzene molecule as a testbed [Eriksen et al., J. Phys. Chem. Lett., 11, 8922 (2020)]. In the process, we review the scope of applications for which FCI continues to prove indispensable, and the required traits in terms of robustness, efficacy, and reliability its modern approximations must satisfy are discussed. We close by conveying a number of general observations on the merits offered by the state-of-the-art alongside some of the challenges still faced to this day. While the field has altogether seen immense progress over the years - the past decade, in particular - it remains clear that our community as a whole has a substantial way to go in enhancing the overall applicability of near-exact electronic structure theory for systems of general composition and increasing size.