Half or full core hole in density functional theory X-ray absorption spectrum calculations of water?

Abstract

It was recently suggested, based on x-ray absorption (XAS) and x-ray Raman spectroscopy (XRS) combined with Density Functional Theory (DFT) calculations of spectra, that water in the liquid is in an asymmetric hydrogen bonding situation with only two strong H-bonds: one donating and one accepting with the remaining bonds weakened or broken through mainly bending off the H-bond angle 1 . This provocative result has attracted much attention and debate in the literature. Smith et al. 2 analyzed the temperature dependence of XAS spectra obtained on a liquid microjet and claimed agreement with fourcoordinated water. Their experimental data has, however, been shown to be flawed by saturation effects which invalidate their conclusions 3 . On the theory side Hetenyi et al. 4 computed spectra directly from their ab initio molecular dynamics (AIMD) simulations and claimed agreement with experiment by using a full core hole (FCH) in their spectrum calculations rather than the standard half core hole (HCH). In view of the predominance (80% at room temperature) of single-donor (SD) species reported in ref. 1 this is a surprising and contradictory result with respect to the XAS conclusions since the AIMD simulation only contained 19% SD species being instead dominated by doubledonor (DD), ice-like coordination. In view of the present debate on the structure of water it is important to analyze in depth the validity of the HCH and FCH approximations to the core-hole potential in DFT calculations of XAS spectra. XAS (elsewhere also NEXAFS or XANES) 5 and the corresponding non-resonant x-ray Raman spectroscopy (XRS) 6 are well-established experimental techniques that have recently been extended to measurements of the local electronic structure of liquid water 1,7-11 . Both XAS and XRS show major differences in the spectral profiles between bulk ice and liquid water 1,8,9 ; there is a well-defined pre-edge structure that can be seen in the spectrum of the liquid and is substantially reduced in the bulk ice spectrum; the remaining intensity is mainly due to proton disorder and minor defects 12 . The liquid water spectrum, with strong pre-edge (535 eV) and main-edge (537 eV) features, instead closely resembles that of the ice surface where a dominant fraction of the water molecules in the first half-bilayer has one free O-H bond 1,13 . The spectrum of bulk ice, in which each molecule is tetrahedrally coordinated, is instead characterized by weak main edge (537 eV) and strong post edge (540 eV) features 1,8 ; these experimental differences already indicate that the liquid cannot be dominated by fully coordinated molecules. In ref. 1 density functional theory (DFT) based spectrum calculations using the standard transition potential or HCH approximation were used to interpolate spectra between the two experimental extremes, i.e. bulk and surface of ice. This confirmed the experimental analysis in terms of coordination and furthermore lead to an operational H-bond definition relative to a geometrical cone at each of the donor hydrogens...

Inelastic x-ray scattering of dense solid oxygen: Evidence for intermolecular bonding

“…Oscillator strengths were obtained from the projection of the core wave function to the O 2p projected density of states. This approach has been shown to produce results in reasonable agreement with the experiment (39,41,42). Two thousand electronic states, including occupied orbitals, were generated for the calculation, yielding electronic excitations up to 577 eV.…”

Tetrahedral structure or chains for liquid water

“…1 was further analyzed by simulating the XAS spectra using core excited electronic state estimates from density function theory (DFT), overlayed on putative local hydrogen-bonding configurations of small water clusters that varied between full tetrahedral hydrogen-bonding through to broken donor hydrogen bonds (1). Wernet et al (1) conclude that the configurations most consistent with the appearance of a pre-edge peak in the simulated near-edge fine structure absorption spectra for liquid water [assuming their full core hole density function theories are adequate, which is currently a matter of debate (2,4,5)] are ones in which there are ''broken'' hydrogen bonds. Whether these hydrogen bonds would be classified as broken under other geometric or energetic criteria has been questioned (3,6).…”

“…In principle, x-ray absorption spectroscopy (XAS) is sensitive to local hydrogen-bonding patterns because it primarily probes instantaneous electronic arrangement of waters in the first coordination shell (1)(2)(3). Recent XAS and x-ray Raman scattering experiments on liquid water have been interpreted to show that the first coordination shell around a water molecule in the bulk fluid has two hydrogen-bonding partners on average (1,2).…”

“…Recent XAS and x-ray Raman scattering experiments on liquid water have been interpreted to show that the first coordination shell around a water molecule in the bulk fluid has two hydrogen-bonding partners on average (1,2). This result was determined by comparing and contrasting the near-edge fine structure of the XAS spectra for liquid water with the same spectral signatures exhibited by hexagonal ice in the bulk, where tetrahedral hydrogen-bonding is unambiguous, and at a prepared I h surface, which involves a significant fraction (50% or more) of broken hydrogen bonds (1,3).…”

Requirements of first-principles calculations of X-ray absorption spectra of liquid water

“…For water, this issue has been discussed, [8][9][10]57,62 and it has been concluded that exciting half an electron, i.e. using the half-core-hole (HCH) approach, provides the best balance between initial and final state effects.…”