Quantum photochemistry: Direct calculation of diabatic states and semiclassical dynamics

This thesis presents progress in six areas of quantum photochemistry: (i) a new algorithm for carrying out efficient semiclassical dynamical calculations using a trajectory surface hopping method for systems with weakly coupled electronic states, (ii) an improved treatment of electronic coherence and decoherence in electronically nonadiabatic molecular collisions, (iii) calculations of coupled diabatic potential energy surfaces by a multi-reference electronic structure method, (iv) comparison of adiabatic ground-state and excited-state adiabatic surfaces calculated by multi-reference electronic structure methods to those calculated by a single-reference method-of-moments and coupled cluster method, (v) fitting coupled diabatic potential energy surfaces for ammonia, and (vi) photodissociation dynamics for van der Waals complexes of Li···FH and Na···FH. Chapter 1 introduces the topic of semiclassical dynamics for electronically nonadiabatic processes and provides an overview of the calculation and fitting of coupled diabatic surfaces. Chapter 2 describes, a new algorithm called the army ants algorithm for trajectory surface hopping. Chapter 3 presents an algorithm called coherent switching with decay of mixing, which is an improved treatment of electronic coherence in non-Born-Oppenheimer trajectories. Chapter 4 provides a comparison of the completely renormalized equation-of-motion coupled-cluster method with multi-reference quasi degenerate perturbation theory near a conical intersection and along a photodissociation coordinate in ammonia. Chapter 5 presents the calculation and fitting of diabatic potential energy surface of ammonia along with the mapping of a four-dimensional conical intersection seam. Chapter 6 presents a study of the photodissociation dynamics of Li···FH and Na···FH van der Waals complexes.