Choice of potential energy surface for MD-ACF propagation

Determine, within the molecular dynamics–autocorrelation function (MD-ACF) cumulant-expansion framework for computing optical line shapes of defects in solids, whether trajectories should be propagated on the ground-state potential energy surface, the excited-state potential energy surface, or an average of the two, and establish criteria that specify under which conditions each choice yields accurate spectra.

Background

The paper develops an MD-ACF approach that samples the gap-operator autocorrelation via classical molecular dynamics and then constructs spectral densities and line shape functions using a cumulant expansion. A key algorithmic decision is which potential energy surface (PES) to use for propagating the trajectory—either the initial (ground) state, the final (excited) state, or an average of both—when computing emission or absorption spectra.

For the 4H‑SiC divacancy benchmark, the authors find that propagating on the initial state PES gives good agreement with prior theory at low temperature; however, they emphasize that this choice may be system dependent. Although expressions exist in the literature for propagating on the final or average PES, a general prescription is lacking, motivating the need to determine principled criteria for PES selection in MD‑ACF line‑shape calculations.