Decoherence-corrected Ehrenfest molecular dynamics on many electronic states.

Esch, Michael P; Levine, Benjamin G

Decoherence corrections increase the accuracy of mixed quantum-classical nonadiabatic molecular dynamics methods, but they typically require explicit knowledge of the potential energy surfaces of all occupied electronic states. This requirement renders them impractical for applications in which large numbers of electronic states are occupied. The authors recently introduced the collapse to a block (TAB) decoherence correction [M. P. Esch and B. G. Levine, J. Chem. Phys. 152, 234105 (2020)], which incorporates a state-pairwise definition of decoherence time to accurately describe dynamics on more than two electronic states. In this work, TAB is extended by introduction of a scheme for efficiently computing a small number of approximate eigenstates of the electronic Hamiltonian, eliminating the need for explicit knowledge of a large number of potential energy surfaces. This adaptation of TAB for dense manifolds of states (TAB-DMS) is systematically improvable by increasing the number of computed approximate eigenstates. Application to a series of one-dimensional model problems demonstrates that TAB-DMS can be accurate when even a very modest number of approximate eigenstates are computed (four in all models tested here). Comparison of TAB simulations to exact quantum dynamical simulations indicates that TAB is quite accurate so long as the decoherence correction is carefully parameterized.