Atomic additivity of the correlation energy in molecules-an ab initio MP4 and G3 study

• Published in: Molecular physics 2003-05, Vol.101 (9), p.1377-1387
• Main Authors: BARIĆ, DANIJELA, MAKSIĆ, ZVONIMIR B., YÁÑEZ, MANUEL
• Format: Article
• Language: English
• ISSN: 0026-8976; 1362-3028
• DOI: 10.1080/0026897031000092922

It is shown that the closed shell valence electron molecular correlation energy of organic molecules in their ground states is a homogeneous multilinear function of the numbers of neutral atoms in their canonical hybridization state. The additivity is a robust feature, which holds for MP2(fc), MP3(fc) and MP4(fc) model calculations. The latter results obtained on a test set of 91 widely different organic molecules, exhibiting a whole gamut of electronic structure patterns, are excellent as evidenced by the absolute average deviation from the additivity values (AAD) of only 1.4 kcal mol −1 and R 2 = 0.999 93. The maximum absolute deviation (MAD) is 5.3 kcal mol −1 . The additivity formula for the total molecular electron correlation retrieved from G3 calculations also has an excellent performance (AAD = 1.2 kcal mol −1 , R 2 = 0.999 98 and MAD = 7.2 kcal mol −1 ). If it is taken into account that the additivity formulae require only back of the envelope calculations, these results are remarkable indeed, in particular since the G3 correlation energies span a very large range from 180.7 (methane) to 1642.8 (hexafluorocyclopropane) kcal mol −1 . Comparison of the exact electron correlation energies in free atoms with the corresponding average correlation energies in molecules reveals that a substantial increase in the latter provides an important contribution in overcoming a very strong Coulomb repulsion between the nuclei. It is shown that the additivity formulae are useful in detecting some special molecular features such as strong resonance and anti-aromaticity.