Longitudinal and transverse excitations: How nominally forbidden signals can be detected in autocorrelation functions relevant to liquids dynamics

This work provides evidence for the crucial role played by the self-part of functions as important as the dynamic structure factor S(Q, ω) and the spectrum of the transverse current autocorrelation CT(Q, ω) in studies of liquid dynamics. By using ab initio molecular dynamics simulations of liquid Ag, we show that a multimode representation also accounts very well for the simulated self-dynamic structure factor Sself(Q, ω), which provides insight on the single-particle dynamics. In particular, we provide a convincing proof that Sself(Q, ω) is responsible for the observation of transverse-like contributions in a longitudinal quantity such as S(Q, ω). It is particularly significant that an analogous situation was found when considering the transverse current. In fact, its self-part, CT,self(Q, ω), was shown to be accountable for the presence of longitudinal-like modes in a transverse quantity, such as CT(Q, ω). Due to the single-particle origin of the transverse signal in the dynamic structure factor, we also show that the apparent Q-dependence of transverse-like frequencies, as inferred from S(Q, ω), differs from the true dispersion of transverse excitations appropriately derived from CT(Q, ω).