Correlation Energy with Interatomic Distances

In examining figures for the variation of correlation energy with distance, it should be remembered that the definition of correlation energy is somewhat arbitrary and when the large-distance limit is approached d Ec may become a purely formal quantity with little physical meaning for instance, in the case of H2 at R = 00, d Ec is equal to the difference between the exact non-relativistic value and the mean value of the Hartree-Fock energy obtained for (H+ - - H ) and (H H) in a variational 

This suggests the following modification in the criterion for approximate constancy of correlation energy, namely that both the number of electron pairs and the number of nearest-neighbour interbond terms 

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Expressions for the force constant, i.r. absorption frequency, Debye temperature, cohesive energy, and atomization energy of alkali-metal halide crystals have been obtained. Gaussian and modified Gaussian interatomic functions were used as a basis the potential parameters were evaluated, using molecular force constants and interatomic distances. A linear dependence between spectroscopically determined values of crystal ionicity and crystal parameters (e.g. interatomic distances, atomic vibrations) has been observed. Such a correlation permits quantitative prediction of coefficients of thermal expansion and amplitude of thermal vibrations of the atoms. The temperature dependence (295—773 K) of the atomic vibrations for NaF, NaCl, KCl, and KBr has been determined, and molecular dynamics calculations have been performed on Lil and NaCl. Empirical values for free ion polarizabilities of alkali-metal, alkaline-earth-metal, and halide ions have been obtained from static crystal polarizabilities the results for the cations are in agreement with recent experimental and theoretical work. 

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