Computational Methods for Statistical Mechanics: Advances and Applications: Part II

Intense ultrafast-laser pulses induce extreme non-equilibrium conditions in matter. In particular, most of the laser energy is absorbed by the electrons, whereas the atoms typically remain unaffected. Considering a fast electron-equilibration process, the electronic system is traditionally described within density functional theory in the canonical ensemble (N,V,T_e) by a constant temperature and Mermin's functionals. However, the assumption of an infinite heat bath coming from the laser has its limits, which leads to the question whether the highly excited electronic system is not best described theoretically in the microcanonical ensemble (N,V,E) assuming a constant entropy S_e [arXiv:2306.08159]. More important, what consequences does this choice have on the ensuing structural response at longer times? Here, we compared \textit{ab initio} molecular dynamics simulations obtained by describing electrons either in the canonical ensemble or by the microcanonical ensemble, as implemented in CHIVES. Our results for silicon indicate that laser-induced phenomena remain conceptually the same independent of the ensemble choice, namely, the atoms follow the same microscopic pathways. We will also discuss results for graphene and graphite. These findings leave the choice of the right ensemble to excitation parameters, like typical timescales of studied phenomena and/or simulation times. We note that the magnitude of some effects may depend on the ensemble choice.