engleski

Atomic Additivity of the Correlation Energy in Molecules - An Ab Initio MP4 and G3 Study

It' is shown that the valence electron molecular correlation energy of organic molecules is a homogeneous multilinear function of the numbers of neutral atoms in their canon- ical hybridization state. The additivity is a robust feature, which holds for MP2(fc), MP3(fc) and MP4(fc) model calculations. The lat ter results obtained on a test Bet of 91 widely different organic. molecules, exhibiting the whole gamut of electronic structure patterns, are excellent as evidenced by the average absolute deviation from the additivity values (AAD) of only 1.4 kcal/mol and R2 = 0.99993. The maximum absolute devi- ation (M AD) is 5.3 kcal/mol. The additivity formula for the total molecular electron correlation retrieved from G3 calculations has also an excellent performance (AAD = 1.2 kcal/mol, R2 = 0.99998 and MAD = 7.2 kcal/mol). If it is taken into account that the additivity formulas require only back of the envelope calculations, these results are indeed remarkable, particularly since the G3 correlation energy span a vel'Y large range from 180.7 (methane) to 1642.8 (hexafluorocyclopropane) kcal/mol. Comparison of the exact electron correlation energies in free atoms \vith the corresponding average corre- lation energies in molecules reveals that a su bstantial increase in the latter provides an important contribution in overcoming a very strong Coulomb repuIsion between the nu- clei upon the chemical band formation. It is shown that the additivity formulas are useful in detecting so me special molecular features like strong resonance and anti/aromaticity.

-

nije evidentirano