Quasi-diabatic representations of adiabatic potential energy surfaces coupled by conical intersections including bond breaking: A more general construction procedure and an analysis of the diabatic representation

The analytic representation of adiabatic potential energy surfaces and their nonadiabatic interactions is a key component of accurate, fully quantum mechanical descriptions of nonadiabatic dynamics. In this work, we describe extensions of a promising method for representing the nuclear coordinate dependence of the energies, energy gradients, and derivative couplings of N-state adiabatic electronic states coupled by conical intersections. The description is based on a vibronic coupling model and can describe multichannel dissociation. An important feature of this approach is that it incorporates information about the geometry dependent interstate derivative couplings into the fitting procedure so that the resulting representation is quantifiably quasi diabatic and quasi diabatic in a least squares sense. The reported extensions improve both the rate of convergence and the converged results and will permit the optimization of nonlinear parameters including those parameters that govern the placement of the functions used to describe multichannel dissociation. Numerical results for a coupled quasi-diabatic state representation of the photodissociation process NH3+hv -> NH2+H illustrate the potential of the improved algorithm. A second focus in this numerical example is the quasi-diabatic character of the representation which is described and analyzed. Special attention is paid to the immediate vicinity of the conical intersection seam. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4734315]