Excited states, symmetry breaking, and unphysical solutions in state-specific CASSCF theory

TL;DR

This paper explores the existence, properties, and challenges of higher-energy solutions in state-specific CASSCF theory, demonstrating their accuracy for excited states and analyzing issues like unphysical solutions and symmetry breaking.

Contribution

It characterizes the topological features of CASSCF solutions, identifies causes of unphysical solutions, and demonstrates the practical advantages of state-specific methods for excited-state calculations.

Findings

Higher-energy solutions can accurately describe excited states with smaller active spaces.

Unphysical solutions arise from redundant orbitals or symmetry breaking.

State-specific solutions can behave quasi-diabatically or adiabatically in crossing regions.

Abstract

State-specific electronic structure theory provides a route towards balanced excited-state wave functions by exploiting higher-energy stationary points of the electronic energy. Multiconfigurational wave function approximations can describe both closed- and open-shell excited states and avoid the issues associated with state-averaged approaches. We investigate the existence of higher-energy solutions in complete active space self-consistent field (CASSCF) theory and characterise their topological properties. We demonstrate that state-specific approximations can provide accurate higher-energy excited states in $\mathrm{H}_2$ (6-31G) with more compact active spaces than would be required in a state-averaged formalism. We then elucidate the unphysical stationary points, demonstrating that they arise from redundant orbitals when the active space is too large, or symmetry breaking when the active space is too small. Furthermore, we investigate the conical intersection in $\mathrm{CH}_2$ (6-31G) and the avoided crossing in $\mathrm{LiF}$ (6-31G), revealing the severity of root flipping and demonstrating that state-specific solutions can behave quasi-diabatically or adiabatically. These results elucidate the complexity of the CASSCF energy landscape, highlighting the advantages and challenges of practical state-specific calculations.