Assessment of Gaussian-3 and Density Functional Theories for Enthalpies of Formation of C₁−C₁₆ Alkanes

We have examined the performance of Gaussian-3 (G3) theory and six related methods for the calculation of enthalpies of formation of n-alkanes of up to 16 carbons and isoalkanes of up to 10 carbons. We have also examined the accuracy of the B3LYP density functional theory for the n-alkanes. The G3 enthalpies of formation of the n-alkanes have errors of less than 2 kcal/mol compared to experiment. There is a small accumulation of error (0.04 kcal/mol per bond) that increases the deviation with chain length. The effects of conformational averaging on the G3 enthalpies of the n-alkanes are estimated to be small, but are in the direction to reduce the error. The branched alkanes have errors of less than 1 kcal/mol. Four of the variations of G3 theory [G3(MP3), G3(MP2), G3(MP2)//B3LYP, and G3(MP2,CCSD)//B3LYP] also have errors similar to or smaller than G3 theory while two of the variations [G3(CCSD) and G3(MP2,CCSD)] have maximum errors for the n-alkanes of about 2.5 kcal/mol. The B3LYP method does very poorly for the calculation of enthalpies of formation of the larger n-alkanes with an error of over 30 kcal/mol for hexadecane. This suggests that B3LYP has a significant problem with accumulation of errors as the molecular size increases. Several schemes for correcting systematic errors in B3LYP calculations for large molecules are also explored.