Separate state vs. transition state Kohn-Sham calculations of X-ray photoelectron binding energies and chemical shifts

The possibilities of using Kohn-Sham density functional theory to make accu rate predictions of core photoelectron binding energies and chemical shifts are explored through a series of calculations on compounds of different si zes and types. The recent proposal of an 'unrestricted generalized transiti on state' (UGTS) method, for this purpose [D.P. Chong, Chem. Phys. Lett. 23 2 (1995) 486; D.P. Chong, J. Chem. Phys. 103 (1995) 1842], is put to test v s. full separate state optimizations of the ground and core hole states, th at is a 'Delta Kohn-Sham' (Delta KS) method. It is found that while interna l parametrizations in terms of grid and basis set expansions can be well co ntrolled there is still a notable dependency of the absolute binding energi es on the choice of functional for both the UGTS and Delta KS methods. As f or pure ab initio dSCF the former method must still be viewed as an approxi mation of the latter. Keeping these dependencies in mind, Kohn-Sham calcula tions seem to provide a promising tool for predicting binding energies and chemical shifts of an accuracy that approaches that of experiments. (C) 199 9 Elsevier Science B.V. All rights reserved.