Nonadiabatic dynamics: A comparison of surface hopping direct dynamics with quantum wavepacket calculations

A semiclassical direct dynamics method for nonadiabatic systems is tested by comparing with quantum wave packet dynamics, looking at the molecular dynamics of the butatriene molecule after formation of the radical cation in the first excited, (A) over tilde, state. There is a conical intersection coupling this state to the cationic ground state,(X) over tilde and this plays a major role in the system evolution. The direct dynamics study consists of 80 trajectories, with the potential energy surfaces calculated on-the-fly using a complete active space (CASSCF) electronic wave function. The quantum dynamics used a model Hamiltonian and the multiconfigurational time-dependent Hartree (MCTDH) method to solve the time evolution of the nuclear wave packet. The results show that the methods give a similar description of the initial part of the dynamics, with a similar time scale for the interstate crossing. A qualitatively different behavior is, however, seen after crossing to the lower adiabatic surface, with a recurrence in the quantum dynamics not present in the direct dynamics. The direct dynamics also indicates the possible importance of a second intersection seam, which is not present in the model used for the quantum dynamics.