Proton transfer in solution: Molecular dynamics with quantum transitions

Abstract

We apply "molecular dynamics with quantum transitions" (MDQT), a surface-hopping method previously used only for electronic transitions, to proton transfer in solution, where the quantum particle is an atom. We use full classical mechanical molecular dynamics for the heavy atom degrees of freedom. including the solvent molecules, and treat the hydrogen motion quantum mechanically. We identify new obstacles that arise in this application of MDQT and present methods for overcoming them. We implement these new methods to demonstrate that application of MDQT to proton transfer in solution is computationally feasible and appears capable of accurately incorporating quantum mechanical phenomena such as tunneling and isotope effects. As an initial application of the method, we employ a model used previously by Azzouz and Borgis to represent the proton transfer reaction AH-B-.=.A --H+B in liquid methyl chloride, where the AH-B complex corresponds to a typical phenol-amine complex. We have chosen this model, in part, because it exhibits both adiabatic and diabatic behavior, thereby offering a stringent test of the theory. MDQT proves capable of treating both limits, as well as the intermediate regime. Up to four quantum states were included in this simulation, and the method can easily be extended to include additional excited states, so it can be applied to a wide range of processes, such as photoassisted tunneling. In addition, this method is not perturbative, so trajectories can be continued after the barrier is crossed to follow the subsequent dynamics.

A general method to describe intersystem crossing dynamics in trajectory surface hopping

“…(Hammes-Schiffer-Tully scheme [155] ). Using the local diabatization scheme, [181,182] the propagator is, instead, given by …”

“…The original surface hopping method was formulated to consider only nonadiabatic couplings [128,155] between electronic states of the same multiplicity and has been therefore intensively used to describe excited state dynamics involving IC. By incorporating SOC into the surface hopping procedure, dynamical studies can be extended to also describe ISC.…”

“…[132,[136][137][138][139][140][141][142][143][144][145][146][147][148][149][150][151][152][153] Tunneling effects can also not be described with surface hopping although there exist several approaches to alleviate this deficiency, see, for example, Refs. [154][155][156][157][158].…”

Efficient real-space configuration-interaction method for the simulation of multielectron mixed quantum and classical nonadiabatic molecular dynamics in the condensed phase

“…With the multielectron eigenstates determined by CI, the solvent dynamics can be propagated using any of a myriad of adiabatic 1,6 or nonadiabatic 1,5,13,21,22 dynamical schemes. Because molecular dynamics simulations require evaluation of multielectron wave functions for thousands of configurations, to make the simulations practical we will also introduce a trick to reduce the computational effort needed for solving the multielectron Schrödinger equation.…”

“…This is because nonadiabatic dynamics requires the full many-electron wave function for both the ground and excited states. Thus, condensed phase systems studied with nonadiabatic dynamics, such as solvated electrons, [8][9][10][11] proton transfer, 12,13 charge-transfer-tosolvent ͑CTTS͒, 14 -16 and donor-acceptor electron transfer complexes, 17 typically are simulated with only a single quan-tum degree of freedom. 18 This restriction to a single quantum degree of freedom is unfortunate because there are several hints that one-electron treatments do not always properly describe the electronic structure of solvated systems.…”