Ab initio Hartree-Fock calculations of molecular X-ray intensities. Validity of one-center approximations

Ab initio Hartree-Fock calculations of relative X-ray transition probabilities for core hole states of N₂, CO, H₂O and NH₃ have been carried out and compared with the respective high-resolution soft X-ray spectra. The same one-determinental wavefunctions were employed for both initial and final states and the dependence of the X-ray transition moments on the choice of orbitals and of basis set parameters was investigated. In particular, orbitals optimized for a transition state were tested. The use of the one-center intensity model as a guide for the assignment of second row X-ray spectra was justified at bothab initio and semiempirical (CNDO) levels of approximation. The breakdown of the MO-picture for inner-valence electrons is demonstrated in the X-ray spectrum of N₂ and the analogy with the corresponding photoelectron bands is pointed out.