Performance of ab initio and density functional methods for conformational equilibria of CnH2n+2 alkane isomers (n=2-8)

TL;DR

This study evaluates the accuracy of various ab initio and density functional methods in predicting conformational energies of alkane isomers, highlighting the superior performance of double-hybrid functionals with dispersion corrections.

Contribution

It provides a comprehensive assessment of DFT and ab initio methods for alkane conformations, establishing double-hybrid functionals as reliable standards.

Findings

Double-hybrid functionals match CCSD(T) quality for conformational energies.

Popular DFT functionals tend to overestimate or underestimate energies without corrections.

PW6B95 and BMK functionals show fewer deficiencies in conformer geometries.

Abstract

Conformational energies of n-butane, n-pentane, and n-hexane have been calculated at the CCSD(T) level and at or near the basis set limit. Post-CCSD(T) contribution were considered and found to be unimportant. The data thus obtained were used to assess the performance of a variety of density functional methods. Double-hybrid functionals like B2GP-PLYP and B2K-PLYP, especially with a small Grimme-type empirical dispersion correction, are capable of rendering conformational energies of CCSD(T) quality. These were then used as a `secondary standard' for a larger sample of alkanes, including isopentane and the branched hexanes as well as key isomers of heptane and octane. Popular DFT functionals like B3LYP, B3PW91, BLYP, PBE, and PBE0 tend to overestimate conformer energies without dispersion correction, while the M06 family severely underestimates GG interaction energies. Grimme-type dispersion corrections for these overcorrect and lead to qualitatively wrong conformer orderings. All of these functionals also exhibit deficiencies in the conformer geometries, particularly the backbone torsion angles. The PW6B95 and, to a lesser extent, BMK functionals are relatively free of these deficiencies. Performance of these methods is further investigated to derive conformer ensemble corrections to the enthalpy function, $H_{298}-H_0$, and the Gibbs energy function, ${\rm gef}(T)\equiv - [G(T)-H_0]/T$, of these alkanes. While $H_{298}-H_0$ is only moderately sensitive to the level of theory, ${\rm gef}(T)$ exhibits more pronounced sensitivity. Once again, double hybrids acquit themselves very well.