posted on 2024-01-04, 12:35authored byEva Vandaele, Momir Mališ, Sandra Luber
A methodology to
locally characterize conical intersections (CIs)
between two adiabatic electronic states for which no nonadiabatic
coupling (NAC) vectors are available is presented. Based on the Hessian
and gradient at the CI, the branching space coordinates are identified.
The potential energy surface around the CI in the branching space
is expressed in the diabatic representation, from which the NAC vectors
can be calculated in a wave-function-free, energy-based approach.
To demonstrate the universality of the developed methodology, the
minimum-energy CI (MECI) between the first (S1) and second (S2) singlet
excited states of formamide is investigated at the state-averaged
complete active space self-consistent field (SA-CASSCF) and extended
multistate complete active space second-order perturbation theory
(XMS-CASPT2) levels of theory. In addition, the asymmetrical MECI
between the ground state (S0) and S1 of cyclopropanone is evaluated
using SA-CASSCF, as well as (ME)CIs between the S1 and S2 states of
benzene using SA-CASSCF and time-dependent density functional theory
(TDDFT). Finally, a CI between the S1 and S2 excited states of thiophene
was analyzed using TDDFT.