Calculations of near-edge x-ray-absorption spectra of gas-phase and chemisorbed molecules by means of density-functional and transition-potential theory

Abstract

We explore the utility of density-functional theory ͑DFT͒ in conjunction with the transition-potential ͑TP͒ method to simulate x-ray-absorption spectra. Calculations on a set of small carbon-containing molecules and chemisorbed species show that this provides a viable option for obtaining excitation energies and oscillator strengths close to the experimental accuracy of core-valence transitions. Systematic variations in energy positions and intensities of the different spectra in the test series have been investigated, and comparison is made with respect to the static exchange-, self-consistent-field, and explicit electron-correlation methods. The choice between standard exchange-correlation functionals is shown to be of little consequence for the valence resonant, here *, parts of the x-ray-absorption spectra, while the long-range behavior of presently available functionals is found not to be completely satisfactory for Rydberg-like transitions. Implementing a basis set augmentation technique, one finds that DFT methods still account well for most of the salient features in the near-edge x-ray-absorption spectra, save for the multielectron transitions in the near continuum, and for some loss of Rydberg structure. For clusters modeling surface adsorbates, the DFT transition potential method reproduces well the spectral compression and intensity reduction for the valence level absorption compared to the free phase, provided fairly large clusters are taken into account. While for near-edge x-ray-absorption fine-structure ͑NEXAFS͒ spectra of free molecules the DFT-TP and Hartree-Fock/static exchange methods have complementary advantages, the DFT-TP method is clearly to be preferred when using clusters to simulate NEXAFS spectra of surface adsorbates. ͓S0163-1829͑98͒00136-2͔

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

“…Within Slater transition-state DFT the core-hole is prepared with half an electron and explicit excited states are formed by adding the excited half electron in the various states. This can be shown to give relaxation effects correct to second order, but as this involves explicit treatment of each and every excited state, the TP-DFT method has been developed, 10,60 in which the excited half electron is neglected and the excited states are obtained from a single matrix diagonalization. However, as a result of this approximation, the degree by which the core-hole should be filled cannot any longer be rigorously decided.…”

ERKALE—A flexible program package for X‐ray properties of atoms and molecules

“…However, the XRS spectrum is also dependent on the momentum transfer, which can be used to extract supporting information not available in XAS experiments. [117][118][119][120] XAS and XRS spectra are computed within ERKALE in the TP approximation, [121] which is based on Slater's transition state approximation. [122,123] The same approach is also available in, for example, the StoBe-deMon, [124] CP2K, [37,38] and the GPAW [15] codes for modeling XAS.…”

High resolution NEXAFS of perylene and PTCDI: a surface science approach to molecular orbital analysis

“…In recent years, the NEXAFS simulation codes have been applied to an increasing number of aromatic systems computed via density functional theory, DFT, most often within the transition-potential method, 22 where the electronic structure is computed in the presence of a half core hole, HCH, at the excited atom. The good agreement with measured spectra [23][24][25][26] justifies the use of these models for the interpretation of experimental data, as compared to a quantum chemistry approach.…”