Center for Functional Nanomaterials Seminar

Talk: Water is arguably the most important material on earth. Oxygen K-edge X-ray absorption spectra (XAS) provide a sensitive local probe of the H-bond structure of liquid water. Based on the static COHSEX approach, we computed the XAS spectra of liquid water from molecular structures generated by ab initio molecular dynamics (AIMD) simulations using a van der Waals (vdW) inclusive hybrid functional (PBE0) that gives ambient water structure in quantitative agreement with experiment. We found that excellent agreement between experimental and theoretical XAS requires both improved molecular modeling and electron excitation treatment. In our simulation the over-structured H-bond network resulting from GGA-AIMD is systematically reduced by as the directional H-bond strength is lowered by the mitigated self-interaction error in PBE0 and the increased population of interstitial water molecules promoted by vdW interactions. The better H-bond structure in turn gives improved XAS spectra. Moreover, we find that the orbitals obtained from the self-consistent diagonalization of the self-energy are crucial in obtaining spectra that compare well with experiment. A perturbative treatment according to the G0W0 approximation leads instead to XAS spectra similar to those obtained by the full core-hole approximation, in which the electron excitations are approximated by the Kohn-Sham conduction band orbitals.