QM–MM Ehrenfest dynamics from first principles: photodissociation of diazirine in aqueous solution

This article describes an implementation of Ehrenfest molecular dynamics based on TDDFT and Gaussian basis sets, optimized for hybrid QM–MM simulations in GPU. The present method makes use of the equations of motion proposed by Chen et al. (J Chem Phys 135:044126, 2011 ), which, at variance with previous formulations of the Ehrenfest dynamics, takes into account the movement of the localized basis functions, thus improving accuracy and energy conservation. This methodology is used to explore the evolution and the stability of excited state dynamics for two different constructions of the initial excited state, consisting in the linear response TDDFT S_1 solution, and in the ground state density matrix where the HOMO–LUMO occupancies have been switched, which is a widespread approach to model photoexcitation in electron dynamics simulations. It is found that the second kind of starting state leads to a larger numerical noise and to a poorer stability of the dynamics, aside from “awakening” inner electronic modes that become manifest in the frequency spectrum, and which are absent if the dynamics departs from the linear response TDDFT density matrix. Then, the method is applied to investigate the photodissociation of the diazirine molecule, CH _2 N _2 , both in vacuum and in aqueous solution. Diazirine decomposes into carbene and molecular nitrogen upon irradiation with UV light, and for this reason it has been widely adopted to photolabel biomolecules through the insertion of carbenes in the macromolecular surface. Our simulations suggest that the quantum yield of the dissociative reaction experiences a decrease in solution with respect to the gas phase, that can be understood in terms of the vibrational relaxation facilitated by the solvent molecules. Besides, the present results indicate that the isomerization and dissociation mechanism occur fully on the S_1 excited state.