Is there any group additive rules in the calculation of electron correlation energies of long straight chain alkane molecules?

Abstract

According to the definition in the text, the correlation energy of 1s2C of carbon atoms, the primary and secondary C-H bonding electron pairs in some CH3, CH2 fragments and CH3(CH2)mCH3 (m=1-5) linear alkane molecules are calculated and analyzed. The transferability of the correlation energies of these electron pairs in the linear alkanes is investigated. The results indicate that the correlation energy of 1s2C is perfectly transferable in the respective methyl and methylene groups, while the correlation energies of the primary and secondary C-H bonding electron pairs are approximately transferable in methyl and methylene groups. The analysis of the results of group correlation energy shows that both of the correlation energies of methyl and methylene groups are transferable in these linear alkanes. The correlation energies of methylene group in CH3(CH2)mCH3 (m=1-5) molecules are slightly decreasing showing a converging trend to a "standard" methylene group in linear alkanes. The excellent fitting relationship between the total correlation energy and the number of methylene groups of the linear alkanes shows that the total correlation energy is a linear function of the number of methylene groups, which means that the total correlation energies of large linear alkanes can be reproduced and predicted by counting the numbers of methylene groups. In this way, total correlation energy of large linear alkane molecule can be approximately calculated using this simple group additive scheme with substantial saving in computational time.