We introduce an interface between PySpawn, a simulation
package
to run ab initio multiple spawning (AIMS) nonadiabatic dynamics, and
OpenMolcas, a software package to perform multiconfigurational perturbations
theory (CASPT2) electronic structure calculations. Our interface allows
us to exploit all the functionalities of the two codes: the modular
and efficient Python implementation of the AIMS algorithm and the
extensive analysis tools offered by PySpawn, with the cutting-edge
implementation of CASPT2 equations in OpenMolcas, including the recently
introduced analytical gradients and different flavors. Both are fully
open-source and free of charge, making the following implementation
unique in the current plethora of software for nonadiabatic dynamics.
This represents an important step toward a wider application of AIMS-based
nonadiabatic dynamics combined with high-accuracy excited-state calculations.
The importance and the need for such an implementation are demonstrated
by application to the ultrafast relaxation of fulvene from S1 to S0, which is drastically affected by the potential
energy surface on which the nuclear wavepacket is propagated. Additionally,
the decay is influenced by the CASPT2 flavor adopted, posing interesting
questions in the choice of one over the other and opening the door
to deeper studies on the effect of CASPT2 formulations in nonadiabatic
dynamics.