A Quick Estimate of the Correlation Energy for Alkanes

Within the localized molecular orbital description, the intra-and interorbital pair correlation energies calculated with the coupled cluster doubles (CCD) theory have been obtained for methane, ethane, propane, butane, isobutane, pentane, isopentane and neopentane using the 6–31G* basis set. The results showed the quantitative transferability of pair correlation energies and gross orbital correlation energies within this series of molecules. Based on the gross orbital correlation energies of five sample alkanes (butane, isobutane, pentane, isopentane and neopentane), we have derived a simple linear relationship to estimate the CCD correlation energy for an arbitrary large alkane. The correlation energy predicted by this simple relationship remarkably recovers more than 98.9% of the exact CCD correlation energy for a number of alkanes containing six to eight carbon atoms. The relative stability of less branched isomers can be correctly predicted.