Multistate, multichannel coupled diabatic state representations of adiabatic states coupled by conical intersections. CH2OH photodissociation.

A coupled diabatic state representation, Hd, of the 1, 2, 3 2A states of CH2OH suitable for the description of the three channel, three state photodissociation process CH2OH(1 2A) + hv → CH2OH(2, 3 2A) → CH2O(X, A) + H, cis-CHOH + H, trans-CHOH + H, is reported. The representation is based on electronic structure data (energies, energy gradients, and derivative couplings) obtained exclusively from multireference configuration interaction single and double excitation wave functions. Diabat shifting is employed to improve the representation's agreement with accurate experimental energetics. A careful analysis of the numerous minima, saddle points, and conical intersection seams is reported. The computed T0(3 2A) ∼ 35 220 cm-1 is in excellent agreement with the experimental estimate of 35 053 cm-1, and the computed channel dissociation energies, D0, for CH2O 9453 (10 160), cis-HCOH 30 310.2 (29 923), and trans-HCOH 28 799 (28 391) cm-1 are in good accord with the measured values given parenthetically. These accurate energetics over a wide range of nuclear configurations strongly support the ability of this Hd to enable quality simulations of nonadiabatic dynamics.