Photochemistry in a dense manifold of electronic states: Photodissociation of CH2ClBr

Abstract

We report electronically nonadiabatic dynamics calculations including spin-orbit coupling for the photodissociation of CH(2)ClBr to yield Cl((2)P(3∕2)), Cl((2)P(1∕2)), Br((2)P(3∕2)), and Br((2)P(1∕2)). The potential energy is a 24 × 24 matrix (divided up here into four 6 × 6 blocks in a first approximation to the problem), in a spin-coupled fully diabatic representation obtained by combining the spin-free fourfold way with single-center spin-orbit coupling constants. The spin-free calculations are carried out by multiconfiguration quasidegenerate perturbation theory, and the fully diabatic potentials including spin-orbit coupling are fit to a matrix reactive force field. The dynamics are carried out by the coherent switches with decay of mixing method in the diabatic representation. The results show qualitative agreement with experiment.

Diabatization based on the dipole and quadrupole: The DQ method

“…33 It has been presented previously [15][16][17][18] and applied successfully to a number of problems. [34][35][36][37][38][39][40] We will also apply it here for comparison with the new DQ method. In this section, we review the key elements of the fourfold way.…”

“…The fourfold-way diabatization has been found to work for all systems to which we have tried to apply it. [15][16][17][18][34][35][36][37][38][39][40] The main drawback is that the proper choice of reference orbitals and the determination of prototype CSFs for the configurational uniformity step require system-dependent decisions that can be timeconsuming and may require expert knowledge of the system at hand. The latter is not entirely unexpected since practical methods involving multiconfiguration reference functions and excited states usually require system-dependent expertise and often require delicate choices about how to treat the various orbitals.…”

Model space diabatization for quantum photochemistry

“…29 It has been successfully applied to a variety of systems at both the CASSCF and MC-QDPT levels. 26,[43][44][45][46][47][48] However, as mentioned in the Introduction, for some cases, the original fourfold way at the MC-QDPT level may give results that are quite different from the standard MC-QDPT method. These problematic cases are the motivation for developing the present MSD strategy.…”

“…In the original paper on applying the fourfold way with MC-QDPT, 19 we pointed out the dependence of MC-QDPT energies on orbital rotations, and we resolved the ambiguity by defining the adiabatic energies as those calculated using the DMOs rather than the canonical molecular orbitals (CMOs). In the original work and subsequent work 26,[43][44][45][46][47][48] we found, when we checked, only small differences between the two sets of adiabatic energies; however, for a current application to thioanisole, we found differences of up to 0.8 eV when using DMOs obtained by the fourfold way and up to 0.05 eV when using DMOs obtained by the threefold way. Therefore, we developed the scheme presented here to give a diabatic potential energy matrix that, when diagonalized, gives precisely the adiabatic energies of standard QDPT with CMOs.…”

Nonadiabatic quantum molecular dynamics with detailed balance

“…Nonadiabatic couplings could potentially affect dynamical ionic properties even at equilibrium; without doubt, nonadiabatic couplings must be accounted for to calculate quantities of current experimental interest, such as temperature equilibration rates [13,14]. The list is not limited to bulk systems, as finite systems can also display a dense manifold of electronic states [15] (see [2] for an extensive list).…”