First-order correction for bond energy applied to polar molecules: Alkali halides, alkali cyanides, LiCH3, and CH3F

The ionic bond in molecules containing an electropositive moiety and an electronegative moiety originates from a coupling between the ionic and the covalent contributions. Some representative cases in the example of LiF, LiCl, NaF, NaCl, KF, KCl, LiCH3, CH3F, LiCN, NaCN and KCN are calculated by ab initio and density functional methods. The resulting bond energy can be improved a posteriori by a recently proposed first-order method using the dipole moment or the effective charge. For the ab initio calculations, this method brings about a systematic improvement of the bond energy with respect to the experimental value. While the density functional method gives qualitatively mixed results, application of this first-order method generally improves the bond energy.