On the description of conical intersections between excited electronic states with LR-TDDFT and ADC(2)

TL;DR

This study evaluates the effectiveness of LR-TDDFT and ADC(2) methods in accurately describing conical intersections between excited states, highlighting their limitations and behaviors in specific molecular cases.

Contribution

The paper provides a detailed analysis of how AA LR-TDDFT and ADC(2) perform in modeling excited-state conical intersections, especially between excited states, using specific molecules as examples.

Findings

LR-TDDFT can depict intersection rings as seams or cones depending on distortions.

ADC(2) shows limitations in accurately describing excited-state intersections.

The methods' performance varies with molecular distortions and intersection types.

Abstract

Conical intersections constitute the conceptual bedrock of our working understanding of ultrafast, nonadiabatic processes within photochemistry (and photophysics). Accurate calculation of potential energy surfaces within the vicinity of conical intersections, however, still poses a serious challenge to many popular electronic structure methods. Multiple works have reported on the deficiency of methods like linear-response time-dependent density functional theory within the adiabatic approximation (AA LR-TDDFT) or algebraic diagrammatic construction to second-order (ADC(2)) - approaches often used in excited-state molecular dynamics simulations - to describe conical intersections between the ground and excited electronic states. In the present study, we focus our attention on conical intersections between excited electronic states and probe the ability of AA LR-TDDFT and ADC(2) to describe their topology and topography, using protonated formaldimine and pyrazine as two exemplar molecules. We also take the opportunity to revisit the performance of these methods in describing conical intersections involving the ground electronic state in protonated formaldimine - highlighting in particular how the intersection ring exhibited by AA LR-TDDFT can be perceived either as a (near-to-linear) seam of intersection or two interpenetrating cones, depending on the magnitude of molecular distortions within the branching space.