Use of partial löwdin (PLA) and limited expansion of diatomic overlap (LEDO) integral approximation methods in molecular orbital calculations of the isomer pairs HCN, CNH and FCN, CNF

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Abstract
Molecular orbital calculations, using a free atom double zeta basis set and embodying the PLA and LEDO integral approximation methods, are performed on the isomer pairs HCN, CNH and FCN, CNF in their respective linear configurations. For each integral approximation method, the C—N separation is fixed at the experimental value for FCN and, for variable X—C and X—N separations, the calculated total energies, orbital energies, one-electron density functions and natural hybrid populations are compared for each pair of isomers. One ab initio calculation is also performed for each isomer at a selected molecular geometry. Additional calculations are performed on the related model ionic systems H+CN– and CN–H+ to provide further insight into the bonding in the two hydrocyano species. Overall, the LEDO method yields a better simulation of ab initio calculations in both absolute and relative terms; but when the results for closely related species are used in a comparative way, the simpler PLA method is also found to provide a surprisingly reliable estimate of the trends in calculated molecular properties.

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