Density functional calculation of K-shell spectra of small molecules

Both ΔEKS and time-dependent density functional theory (TD-DFT) methods, with approximations for the singlet-triplet splitting and for the relativistic corrections, were tested for the calculation of K-shell spectra of Ne, HF, H2O, NH3, CH4, and CO. Results from several exchange-correlation functionals as well as diffuse basis sets were compared with available experimental data. Excellent core excitation and core-electron ionization energies for Ne, HF, H2O, NH3, CH4, and CO can be obtained from ΔE with Perdew-Wang 1986 exchange and Perdew-Wang 1991 correlation functionals; and reasonable intensities for singlet excitations, from TD-DFT with exchange-correlation potential known as statistical average of orbital potentials. The dependence of the quality of ΔE on basis set is as expected: excitations to higher Rydberg levels requiring more diffuse functions. However, the oscillator strength seems to be more sensitive to the quality of the basis set. Suggestions are made for extending the procedure to larger systems.