Core excitation energies calculations

Table 14.2 summarizes CO core-excitation energies calculated by the ASCF and SIC-ASCF methods using HF and KS-DFT with BLYP, B3LYP, and BHHLYP. 

In time-dependent Hartree-Fock (TDHF) and TDDFT calculations, the selfinteractions for unoccupied orbitals are automatically corrected. Here, in order to remove the SIEs of occupied orbitals, the following modified A matrix is adopted to estimate SIC core-excitation energies ... 

The orbitals from which electrons are removed can be restricted to focus attention on the correlations among certain orbitals. For example, if the excitations from the core electrons are excluded, one computes the total energy that contains no core correlation energy. The number of CSFs included in the Cl calculation can be far in excess of the number considered in typical MCSCF calculations. Cl wavefimctions including 5000 to 50 000 CSFs are routine, and fimctions with one to several billion CSFs are within the realm of practicality [53]. 

HyperChem supports MP2 (second order Mpller-Plesset) correlation energy calculations using any available basis set. In order to save main memory and disk space, the HyperChem MP2 electron correlation calculation normally uses a so called frozen-core approximation, i.e. the inner shell (core) orbitals are omitted. A setting in CHEM.INI allows excitations from the core orbitals to be include if necessary (melted core). Only the single point calculation is available for this option. 

Levin et a/.194 have calculated the GVB rc-orbitals of C2H4 using a fixed (7-core which was obtained from a full HF calculation. The orbitals were spatially projected to obtain the correct spatial and spin symmetry without restricting the nature of the individual orbitals. The agreement with the results of full Cl calculations was good for both total energies and excitation energies. 

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