Modified valence indices from the two‐particle density matrix

The modified ionic and covalent valence indices are introduced, defined in the framework of the two‐particle density matrix, with respect to the reference state of separated atoms or ions ( SAL ). They include only quadratic contributions in changes of the molecular charge‐and‐bond order matrix elements, relative to the SAL . General properties of the modified valence indices are examined and illustrative qualitative results for model systems are presented. Numerical UHF SCF MO valence data for selected diatomic and triatomic molecules are reported and interpreted in terms of the valence saturation effect and the ionic vs. covalent valence competition. A three‐orbital valence model of a symmetric transition state of the bond‐forming–bond‐breaking reaction supports the BEBO model postulate of preservation of the total “bond order.” The model predictions are compared with the UHF numerical values.