Ultrafast Near-Edge X-ray Absorption Fine Structure Calculations with the Exact Integral Simplified Time-Dependent Density Functional Theory (XsTD-DFT) for Large Systems

Computing a near-edge X-ray absorption fine structure (NEXAFS) is a real challenge for quantum chemistry (QC), as for medium to large systems, it involves a high density of core-valence excited states. With the boundaries of QC pushed at its maximum with the exact integral simplified time-dependent density functional theory (XsTD-DFT) framework, an ultrafast method is proposed to compute such excitations with short-range corrected exchange-correlation functionals using the Tamm-Dancoff approximation. For small to medium size systems, computations were performed in less than a minute, providing striking comparisons with respect to the experiment. To showcase the performance of the method, the computed oxygen K-edge NEXAFS spectrum for a collagen model of 600 atoms was compared to the experimental spectrum of collagen. Computing 85 672 1s_O core-valence excited states was necessary to reproduce the experimental spectrum. The calculation took only 11 days on a desktop computer. With knowledge of the simplicity of this "small" static model of collagen, the comparison to the experiment remains excellent.