posted on 2020-03-23, 15:03authored byYu Zhang, Linqiu Li, Sergei Tretiak, Tammie Nelson
Nonadiabatic
Molecular Dynamics (NAMD) of excited states has been
widely used in the simulation of photoinduced phenomena. However,
the inability to treat bond breaking and forming processes with single-reference
electronic structure methods limits their application in photochemistry
for extended molecular systems. In this work, the extension of excited-state
NAMD for open-shell systems is developed and implemented in the NEXMD
software. We present the spin-unrestricted CIS and TD-SCF formalism
for the ground and excited states, analytical derivatives, and nonadiabatic
derivative couplings for the respective potential energy surfaces.
This methodology is employed to study the photochemical reaction of
three model molecules. The results demonstrate the advantage of the
open-shell approach in modeling photochemical reactions, especially
involving bond breaking processes. We find that the open-shell method
lowers the reaction barrier at the bond-breaking limits resulting
in larger calculated photochemical quantum yields compared to the
respective closed-shell results. We also address problems related
to spin contamination in the open-shell method, especially when molecular
geometries are far from equilibrium.