Multireference electron correlation methods: Journeys along potential energy surfaces

TL;DR

This review discusses recent advances in multireference electron correlation methods, emphasizing their applications in exploring potential energy surfaces, geometry optimization, and photodynamics, with a focus on analytical gradients and derivative couplings.

Contribution

It summarizes developments over the past two decades in analytical gradients, derivative couplings, and software infrastructure for multireference methods in quantum chemistry.

Findings

Enhanced accuracy in PES exploration and geometry optimization.

Development of analytical nuclear gradients and derivative couplings.

Improved software tools enabling practical applications.

Abstract

Multireference electron correlation methods describe static and dynamical electron correlation in a balanced way, and therefore, can yield accurate and predictive results even when single-reference methods or multiconfigurational self-consistent field (MCSCF) theory fails. One of their most prominent applications in quantum chemistry is the exploration of potential energy surfaces (PES). This includes the optimization of molecular geometries, such as equilibrium geometries and conical intersections, and on-the-fly photodynamics simulations; both depend heavily on the ability of the method to properly explore the PES. Since such applications require the nuclear gradients and derivative couplings, the availability of analytical nuclear gradients greatly improves the utility of quantum chemical methods. This review focuses on the developments and advances made in the past two decades. To motivate the readers, we first summarize the notable applications of multireference electron correlation methods to mainstream chemistry, including geometry optimizations and on-the-fly dynamics. Subsequently, we review the analytical nuclear gradient and derivative coupling theories for these methods, and the software infrastructure that allows one to make use of these quantities in applications. The future prospects are discussed at the end of this review.