Applying generalized variational principles to excited-state-specific complete active space self-consistent field theory

TL;DR

This paper introduces a generalized variational principle for excited-state-specific CASSCF, enhancing stability, convergence, and state-targeting, especially for challenging charge transfer and doubly excited states.

Contribution

The approach improves upon previous methods by resisting root flipping and enabling optimal orbitals for individual excited states, including states previously difficult to locate.

Findings

Demonstrated improved convergence in LiH, ozone, and MgO.

Achieved locating three excited states in MgO that previous methods could not find.

Enhanced stability and state-specific optimization over traditional approaches.

Abstract

We employ a generalized variational principle to improve the stability, reliability, and precision of fully excited-state-specific complete active space self-consistent field theory. Compared to previous approaches that similarly seek to tailor this ansatz's orbitals and configuration interaction expansion for an individual excited state, we find the present approach to be more resistant to root flipping and better at achieving tight convergence to an energy stationary point. Unlike state-averaging, this approach allows orbital shapes to be optimal for individual excited states, which is especially important for charge transfer states and some doubly excited states. We demonstrate the convergence and state-targeting abilities of this method in LiH, ozone, and MgO, showing in the latter that it is capable of finding three excited state energy stationary points that no previous method has been able to locate.