Parametrization of a linear vibronic coupling model with multiconfigurational electronic structure methods to study the quantum dynamics of photoexcited pyrene

With this work we present a protocol for the parametrization of a Linear Vibronic
Coupling (LVC) Hamiltonian for quantum dynamics, using highly accurate multiconfigurational electronic structure methods such as RASPT2/RASSCF, combined with
a maximum-overlap diabatization technique. Our approach is fully portable, and
could be applied to many medium-size rigid molecules whose excited state dynamics
requires a quantum description. We present our model and discuss the details of the
electronic structure calculations needed for the parametrization, analyzing critical
situations that could arise in the case of strongly interacting excited states. The
protocol was applied to the simulation of the excited state dynamics of the pyrene
molecule, starting from either the rst or the second bright state (S2 or S5). LVC
model was benchmarked against state-of-the-art QM calculations with optimizations
and energy scans, and turned out to be very accurate. The dynamics simulations,
performed including all active normal coordinates with the multilayer multiconfigurational
time-dependent Hartree method, show good agreement with the available
experimental data, endorsing prediction of the excited state mechanism, especially
for S5, whose ultrafast deactivation mechanism was not yet clearly understood.