On the validity of the equivalent cores approximation for computing X-ray photoemission and photoabsorption spectral bands

We evaluate the performance of the equivalent cores (Z + 1) approximation for computing core photoemission and photoabsorption spectral bands versus full self-consistent field optimizations. Franck-Condon factors are obtained using the linear coupling model with the multi-dimensional vibronic coupling constants computed from analytical calculations of the ground state frequencies and normal coordinates, and excited state energy gradients evaluated at the ground state equilibrium geometry. We use carbon monoxide, formaldehyde, chloromethanes and some monosubstituted benzenes for the purpose. Considering other inherent approximations in the calculations, we find that the equivalent core model works well for band shapes and comparatively better for excitations of strong modes and ionization of deeper core levels. The approximation also works somewhat better for photoemission than for discrete photoexcitation. (C) 2000 Elsevier Science B.V. All rights reserved.