Simulating photochemistry by nonadiabatic dynamics

Theoretical treatment of nonadiabatic dynamics requires two main ingredients, a reliable method to calculate excited states and a technique to perform molecular dynamics (MD) simulations in two or more coupled electronic states. Car-Parrinello (or ab initio) molecular dynamics (CP-MD) is a technique to simultaneously propagate atomic nuclei and their electrons based on density functional theory (DFT). Until recently this method was applicable only to systems in the electronic ground state. In 1998, a novel CP-MD scheme was introduced that made possible MD simulations in the lowest excited singlet state, S1. We have combined the ground and excited state CP-MD approaches and, in addition, incorporated nonadiabatic effects by implementing a so-called surface hopping algorithm (see Fig. 1). Our nonadiabatic CP-MD (na-CP-MD) method has been tested on a well-known photochemical reaction in the gas phase, namely the cis - trans photoisomerisation of formaldimine (see Fig. 2). Photoisomerisation about a double bond plays an important role in many biological processes such as vision. However, since real-life biochemical reactions usually take place in aqueous solution, it is desirable to be able to carry out nonadiabatic MD simulations of condensed phases. This is where the present na-CP-MD approach is most efficient. In order to demonstrate the applicability of na-CP-MD to condensed phase systems, we have studied the trans - cis photoisomerisation of diazene, N2H2, in aqueous solution (see Fig.3).