Ab initio excited state photodynamical simulations have entered the mainstream in the past two decades, bringing techniques of various sophistication and computational requirements for the description of nonadiabatic transitions. We explore in this work the performance of the recently reformulated Landau−Zener surface hopping (LZSH) approach and extend it for the simultaneous treatment of internal conversion and intersystem crossing events. We studied photochemical reactions of four model molecules (cyclopropanone, methaniminium cation, cytosine, and thiophene). The calculated quantities are generally in excellent agreement with the corresponding fewest switches surface hopping simulations. Furthermore, the algorithm proved to be significantly more stable and more computationally efficient. LZSH also puts fewer constraints on the electronic structure theory as the nonadiabatic couplings are not needed. We argue that the accuracy of photodynamical simulations is in practice dominated by the electronic structure theory, and it is, therefore, legitimate to use the simplest and the most efficient technique for the treatment of nonadiabatic transitions.

Photodynamical simulations are increasingly used to explore photochemical mechanisms and interpret laser experiments. The vast majority of ab initio excited-state simulations are performed within semiclassical, trajectory-based approaches. Apart from underlying electronic-structure theory, the reliability of simulations is controlled by a selection of initial conditions for the classical trajectories. We discuss appropriate choices of initial conditions for simulations of different experimental arrangements: dynamics initiated by continuum-wave (CW) laser fields or triggered by ultrashort laser pulses. We introduce a new technique, CW-sampling, to treat the former case, based on the ideas of importance sampling, combined with the quantum thermostat approach based on the Generalized Langevin Equation (GLE) that allows for efficient sampling of both position and momentum space. The CW-sampling is particularly important for photodynamical processes initiated by absorption at the tail of the UV absorption spectrum. We also emphasize the importance of non-Condon effects for the dynamics. We demonstrate the performance of our approach on the photodissociation of the CF2Cl2 molecule (Freon CFC-12). A quantitative agreement with the experimental data is achieved with the use of empirical correlation energy correction (CEC) factor on top of FOMO-CASCI potential energy surfaces.

The combination of supersonic expansions with IR action spectroscopy techniques is the basis of many successful approaches to study cluster structure and dynamics. In this paper we elucidate the temperature effect of IR excitation and evaporative cooling on sodium solvation in water clusters.