  To date X-ray spectroscopy has become a routine tool that can reveal highly
local and element-specific information on the electronic structure of atoms in
complex environments. Here, we focus on nuclear dynamical effects in X-ray
spectra and develop a rigorous time-correlation method employing ground state
molecular dynamics simulations. The importance of nuclear correlation phenomena
is demonstrated by comparison against the results from the conventional
sampling approach for gas phase water. In contrast to the first-order
absorption, second-order resonant inelastic scattering spectra exhibit
pronounced fingerprints of nuclear motions. The developed methodology does not
depend on the accompanying electronic structure method in principle as well as
on the spectral range and, thus, can be applied to, e.g., UV and X-ray
photo-electron and Auger spectroscopies.
