Abstract
The capability of fewest-switches surface hopping (FSSH) to describe non-adiabatic dynamics under explicit excitation with external fields is evaluated. Different FSSH parameters are benchmarked against multi-configurational time dependent Hartree (MCTDH) reference calculations using SO2 and 2-thiocytosine as model, yet realistic, molecular systems. Qualitatively, FSSH is able to reproduce the trends in the MCTDH dynamics with (also without) an explicit external field; however, no set of FSSH parameters is ideal. The adequate treatment of the overcoherence in FSSH is revealed as the driving factor to improve the description of the excitation process with respect to the MCTDH reference. Here, two corrections were tested: the augmented-FSSH (AFSSH) correction and the energy-based decoherence correction. A dependence on the employed basis is detected in AFSSH, performing better when spin-orbit and external laser field couplings are treated as off-diagonal elements instead of projecting them onto the diagonal of the Hamilton operator. In the presence of an electric field, the excited state dynamics was found to depend strongly on the vector used to rescale the kinetic energy along after a transition between surfaces. For SO2, recurrence of the excited wave packet throughout the duration of the applied laser pulse is observed for laser pulses (>100 fs), resulting in additional interferences missed by FSSH and only visible in variational multi-configurational Gaussian when utilizing a large number of Gaussian basis functions. This feature vanishes when going toward larger molecules, such as 2-thiocytosine, where this effect is barely visible in a laser pulse 200 fs long.
Highlights
Femtosecond time-resolved spectroscopy has progressed drastically throughout the past decades,1 challenging the computational excited state dynamics simulations to explicitly include laser pulses.2–12 Following a laser excitation to some high-lying electronic state, a wave packet can evolve through different potential energy surfaces (PESs), deactivating to the electronic ground state by radiationless or radiative processes
We use a linear vibronic coupling (LVC) Hamiltonian that contains four singlet and three triplet states obtained with multi-reference configuration interaction including single excitations, which is able to reproduce the main features of the full-dimensional excited state dynamics
We benchmark the performance of different parameters that are needed in fewest-switches surface hopping (FSSH) against a multi-configurational time dependent Hartree (MCTDH) reference for two molecular systems: SO2 and 2-thiocytosine using parametrized LVC potentials
Summary
Femtosecond time-resolved spectroscopy has progressed drastically throughout the past decades, challenging the computational excited state dynamics simulations to explicitly include laser pulses. Following a laser excitation to some high-lying electronic state, a wave packet can evolve through different potential energy surfaces (PESs), deactivating to the electronic ground state by radiationless or radiative processes. This paper is spurred on bridging recent investigations that highlighted errors present in FSSH and more apparently in the presence of laser fields and the more pragmatic aim of striving to increase the comparability with experimental data by including laser fields in realistic molecular studies With this in mind, we use two model systems, SO2 and 2-thiocytosine, to test the influence of different FSSH parameters dealing with decoherence, representations, and rescaling options of the kinetic energy after a change between PESs or a frustrated hop. We use two model systems, SO2 and 2-thiocytosine, to test the influence of different FSSH parameters dealing with decoherence, representations, and rescaling options of the kinetic energy after a change between PESs or a frustrated hop These tests are conducted both using an explicit laser pulse to excite the system and the much more common approach of neglecting any external field and just placing the wave packet directly in the optical bright state at the beginning of the dynamics.
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