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 f or computing core photoemission and photoabsorption spectral bands versus f ull self-consistent field optimizations. Franck-Condon factors are obtained using the linear coupling model with the multi-dimensional vibronic coupli ng constants computed from analytical calculations of the ground state freq uencies and normal coordinates, and excited state energy gradients evaluate d at the ground state equilibrium geometry. We use carbon monoxide, formald ehyde, chloromethanes and some monosubstituted benzenes for the purpose. Co nsidering other inherent approximations in the calculations, we find that t he equivalent core model works well for band shapes and comparatively bette r for excitations of strong modes and ionization of deeper core levels. The approximation also works somewhat better for photoemission than for discre te photoexcitation. (C) 2000 Elsevier Science B.V. All rights reserved.