Nonadiabatic dynamics study of methaniminium with ORMAS: Challenges of incomplete active spaces in dynamics simulations

The issues associated with the use of multiconfigurational wave function methods, especially those that use active spaces with less than a full-valence orbital space or incomplete excitations between multiple active spaces or both, in nonadiabatic dynamics is carefully examined. Toward this end, the dynamics package NEWTON-X and the electronic structure suite GAMESS are interfaced for nonadiabatic and adiabatic "on-the-fly" dynamics simulations. In particular, this interface allows for the first study of nonadiabatic dynamics with the occupation restricted multiple active space (ORMAS) approximation, which is unique to GAMESS. Several dynamics simulations using methaniminium as an example were performed with various computationally feasible active space choices (or schemes) in order to test the qualitative accuracy and relative expense of different active space choices. Overall, for ORMAS orbital subspace divisions, schemes with no excitations between orbital subspaces give qualitatively incorrect state populations while schemes with single excitations between orbital subspaces recover the qualitatively correct state populations relative to the CASSCF level of theory at a lower computational expense. However, larger number of excitations between ORMAS subspaces cause more issues with orbital integrity within the active spaces, especially at the initiation of the trajectories. In addition, all active spaces show a large number of trajectories with an orbital integrity issue that is not caught by the energy conservation checks. So, trajectories must be monitored carefully and more overall trajectories are likely to be needed to obtain quantitative statistical information.