Full-dimensional potentials and state couplings and multidimensional tunneling calculations for the photodissociation of phenol

We present an improved version of the anchor points reactive potential (APRP) method for potential energy surfaces; the improvement for the surfaces themselves consists of using a set of internal coordinates with better global behavior, and we also extend the method to fit the surface couplings. We use the new method to produce a 3 x 3 matrix of diabatic potential energy surfaces and couplings for the photodissociation of phenol as functions of 33 nonredundant internal coordinates. The diabatic potential matrix is based on two kinds of calculations at a sequence of anchor points along the O-H dissociation coordinate: (1) fourfold way diabatic calculations based on MC-QDPT/jul-cc-pVDZ calculations for the potential energy surfaces and diabatic couplings as functions of the O-H bond stretch, C-O-H bond angle, and C-C-O-H torsion and for the diabatic couplings as functions of the nine out-of-plane phenoxyl distortion coordinates and (2) M06-2X/jul-cc-pVDZ density functional Hessian calculations for the potentials along the 30 vibrational coordinates of the phenoxyl group. The potential energy surfaces and couplings are used to calculate and characterize adiabatic surfaces and conical intersections, and the resulting equilibrium geometries, vibrational frequencies, and vertical excitation energies are in good agreement with available reference data. We also calculate the geometries of the minimum energy conical intersections. The surfaces and couplings are used for full-dimensional tunneling calculations of the adiabatic photodissociation rate, i.e., the rate of O-H bond fission following photoexcitation. Finally we use the couplings to provide indicators of which vibrational modes are effective in promoting dissociation.