Abstract
The adequate exploration of the phase space of a chromophore is a fundamental necessity for the simulation of their optical and photophysical properties, taking into account the effects of vibrational motion and, most importantly, the coupling with a (non-homogeneous) molecular environment. A representative set of conformational snapshots around the Franck-Condon region is also required to perform non-adiabatic molecular dynamics, for instance in the framework of surface hopping. Indeed, in the latter case one needs to prepare a set of initial conditions providing a meaningful and complete statistical base for the subsequent trajectory propagation. In this contribution, we propose two new protocols for molecular dynamics-based phase space sampling, called “local temperature adjustment” and “individual QM/MM-based relaxation.” These protocols are intended for situations in which the popular Wigner distribution sampling procedure is not applicable—as it is the case when anharmonic or nonlinear vibrations are present—and where regular molecular dynamics sampling might suffer from an inaccurate distribution of internal energy or from inaccurate force fields. The new protocols are applied to the case of phase space sampling of [Re(CO)3(Im)(Phen)]+ (im, imidazole; phen, phenanthroline) in aqueous solution, showing the advantages and limitations of regular Wigner and molecular dynamics sampling as well as the strengths of the new protocols.
Highlights
Photochemical and photophysical phenomena have constantly gained importance in the last decades in many areas of research and technology
We computed the distribution of internal coordinates for the three ensembles that appeared during the adjustment steps: (i) the 500 snapshots taken directly from the 10 ns molecular dynamics (MD) trajectory, (ii) the 500 snapshots obtained after local temperature adjustment (“force field (FF) 600 K”), and (iii) the 500 snapshots produced at the end of the quantum mechanics/molecular mechanics (QM/MM)-MD trajectories (“QM/MM”)
An alternative way to sample the phase space of this complex is based on classical molecular dynamics (MD)
Summary
Photochemical and photophysical phenomena have constantly gained importance in the last decades in many areas of research and technology. From the side of theoretical and computational chemistry, among the most relevant approaches to describe photochemical and photophysical phenomena are the simulation of UV-vis absorption. The information that can be gained through these simulations include the number and character of electronically excited states, their lifetimes, and the quantum efficiency connected to various photochemical reactions (Tully, 1990; Persico and Granucci, 2014). The dynamics simulations allow following the coupled nuclear motion and evolution of the electronic wave functions. In that sense, such simulations provide an approach to study and model photophysical properties that are complementary to experiments such as time-resolved pump-probe spectroscopy (Petit and Subotnik, 2014; Arbelo-González et al, 2016; Ruckenbauer et al, 2016; Crespo-Otero and Barbatti, 2018)
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