An improved quasi-diabatic representation of the 1, 2, 3 1 A coupled adiabatic potential energy surfaces of phenol in the full 33 internal coordinates
In a previous project we constructed a quasi-diabatic representation, H d , of the 1, 2, 3 1 A adiabatic states of phenol from high level multireference single and double excitation configuration interaction electronic structure data, energies, energy gradients, and derivative couplings. That H d accurately describes surface minima, saddle points, and also regions of strong nonadiabatic interactions, reproducing the locus of conical intersection seams and the coordinate dependence of the derivative couplings. The present work determines the accuracy of H d for describing phenol photodissociation. Additionally, we demonstrate that a modest energetic shift of two diabats yields a quantifiably more accurate H d compared with experimental energetics. The analysis shows that in favorable circumstances it is possible to use single point energies obtained from the most reliable electronic structure methods available, including methods for which the energy gradients and derivative couplings are not available, to improve the quality of a global representation of several coupled potential energy surfaces. Our data indicate an alternative interpretation of kinetic energy release measurements near λ phot ~ 248 nm.