Structures of Ground-State Molecules for Time-Averaged GED

To refine time-averaged GED data, experimentalists need a good guess for the gas-phase molecular structure of the species under investigation. The GED data are refined via a least-squares fit of a theoretical geometric model to the experimental GED data; to create this model, it is necessary to determine:

• the equilibrium gas-phase structure with high accuracy
• if and where it is possible to employ symmetry to simplify the geometric model
• the vibrational modes and frequencies (to correct the time-averaged structure for vibrational motion)
• the fractional distribution of conformers present at the temperature of the GED experiment.

Depending on the species under investigation, other properties may require computation to further aid the refinement or to explain the structure/reactivity observed.

Structures of Excited-State Molecules for Time-Resolved GED

To understand electronically-excited states and non-adiabatic effects, in particular, it is often necessary to go beyond single-reference methods; the Wann Electron Diffraction Group uses a range of multireference methods to investigate molecules in their electronically-excited states. To plan a time-resolved GED (TRGED) experiment, it is necessary at least to determine:

• vertical excitation energies from the ground-state equilibrium structure to the Franck-Condon point
• accessible excited-state equilibrium structures
• accessible excited-state transition structures
• the location and topology of accessible minimum-energy conical intersections (MECIs) on the crossing seam between different excited states
• the profile of the excited-state potential surface in multiple dimensions. 

Computing these allows the photodynamics to be qualitatively assessed and possible routes across the excited-state potential surface to be plotted. If the photodynamics appear interesting, a case can be made for full surface-hopping molecular dynamics (SHMD) simulations and future TRGED experiments.