RTXS: A real-time time-dependent density functional package for calculating real time effects in X-ray spectra
Recent developments such as pulsed laser sources for X-ray spectroscopy have improved experimental resolution of time dependent response. These experimental advancements demand corresponding theoretical advances with which to compare. The Rehr group intends to develop and implement a real time formalism, RTXS, for the calculation of X-ray absorption and emission spectra for molecular and nano-scale systems. The current focus is on the real time effect of core hole creation on X-ray absorption spectra.
RTXS uses a fork of RT-SIESTA as a backend, which implements the time-dependent density functional theory (TDDFT) by propagating the DFT wavefunctions in real-time. We calculate the X-ray spectra by taking the Fourier transform of the time correlation function of the photoelectron wavefunction. The propagation uses a predictor-corrector Crank-Nicolson (PCCN) algorithm which ensures the time-reversibility of the approach, resulting in a very stable implementation allowing for long time steps. RTXS has produced some calculations comparable to frequency space results. We are currently developing RTXS to study real time effects of core hole creation. The RT-SIESTA backend is an extension of the popular SIESTA code, and takes advantage of its efficient linear combination of atomic orbitals (LCAO) basis set, and use of pseudopotentials.