Surface-Hopping Molecular Dynamics

As a prelude to time-resolved GED (TRGED) experiments, the Wann Electron Diffraction Group uses surface-hopping molecular dynamics (SHMD) simulations to explore the ultrafast photodynamics of small molecules in the moments immediately following photoexcitation to an electronically-excited state. These simulations give invaluable insight into:

• the type and timescale of photodynamics, important for:
  • assessing the suitability of the molecular species under investigation for fast-tracking to TRGED experiments
  • generating mock TRGED data for the molecule under investigation

• the occurrence of ultrafast non-radiative effects, interesting from a purely theoretical perspective and important for:
  • predicting photoproduct distributions
  • assessing the likelihood and coherency of a photoinduced structural change.

A broad range of interests are represented in the Wann Electron Diffraction Group, spanning biological, interstellar, atmospheric and functional materials chemistry. As such, no 'black box' SHMD simulation exists and the appropriateness of a certain level of calculation is highly system-dependent, although a summary of some common calculations is given below.

Case Study: 1,2-Dithiane

Our recent non-adiabatic multi-configurational SHMD simulations for 1,2-dithiane inspired MeV ultrafast electron diffraction (UED) work at the SLAC National Accelerator Laboratory (Stanford, US). The simulations revealed a molecular "Newton's Cradle" that activates on absorption of light in the mid-UV and assists internal conversion in 1,2-dithiane, protecting the disulphide bond from photodamage. The work challenged contemporary understanding of the photodynamics of 1,2-dithiane and allowed us to make a classically-intuitive reinterpretation of experimental evidence. The work was published in Phys. Chem. in September 2016; read it here!