Excitation energy, first from correlation potentials

First excitation energies determined from ground-state exchange-correlation potentials... 

Table II presents the first excitation energies obtained from spin- polarized calculations [24]. As ground-state exchange-correlation potentials were used the extra term in Eq.(20) does not appear. This is, certainly, one of the reasons for the difference between the calculated and the experimental excitation energies. There is a definite improvement comparing with the nonspin-polarized results [13]. Still, in most cases the calculated excitation energies are highly overestimated. The results provided by the different local density approximations are quite close to each other. The best one seems to be the Gunnarson-Lundqvist-Wilkins approximation. (In non-spin-polarized case the Perdew-Zunger parametrization gives results closest to the experimental data[30].)...

The Bouteiller approach to anharmonicity also permits extraction of energies of excitation to vibrational levels beyond the first excitation. The various progressions are reported in Table 3.19, arising from the correlated potentials. Proceeding down each column, the spacing between successive overtones decreases as the quantum number rises, resulting from the mechanical anharmonicity. 

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