Valence-State Atoms in Molecules. 7. Influence of Polarization and Bond-Charge on Spectroscopic Constants of Diatomic Molecules

The polarizable valence-state-atoms-in-molecules (pVSAM) model describes the electron-pair bond in A−B molecules by superposing core-polarized A + B -, A - B +, and A:B structures, whose weights are determined by electronegativity equalization. The polarizable valence state potential energy curve (pVS-PEC) is derived through the systematic improvement of the valence state potential energy curve (VS-PEC) [Gardner, D. O. N.; von Szentpály, L. J. Phys. Chem. A 1999, 103, 9313] and is given as U ( R ) = −[( K 1 / R) + ( K 2 / R 4 ) + ( K 3 / R 7 )] + ( T / R ) exp(−λ R ). The first bracketed term contains the Coulomb, charge-induced dipole, and induced dipole−induced dipole terms, derived from weighted ionic and covalent bond-charge contributions. The potential is tested on a broad variety of homonuclear diatoms and heteronuclear halides and hydrides (a total of 52 molecules). The accuracies of the dimensionless vibration−rotation coupling constant ( F ) and the anharmonicity constant ( G ) for the halides of the alkali and coinage metals are significantly better than those of the Morse, Rydberg, simple bond-charge, and Rittner potentials. Adding core polarization to the VS-PEC reduces the average unsigned errors in the spectroscopic constants of 47 diatomic molecules from 17.1% to 7.5% in F and 18.9% to 7.8% in G, whereas those of the Morse potential amount to 32.6% and 31.4%, respectively.