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The thermochemistry of linear and branched alkanes with up to eight carbonshas been reexamined by means of W4, W3.2lite and W1h theories. `Quasi-W4'atomization energies have been obtained via isodesmic and hypohomodesmoticreactions. Our best atomization energies at 0 K (in kcal/mol) are: 1220.04n-butane, 1497.01 n-pentane, 1774.15 n-hexane, 2051.17 n-heptane, 2328.30n-octane, 1221.73 isobutane, 1498.27 isopentane, 1501.01 neopentane, 1775.22isohexane, 1774.61 3-methylpentane, 1775.67 diisopropyl, 1777.27 neohexane,2052.43 isoheptane, 2054.41 neoheptane, 2330.67 isooctane, and 2330.81hexamethylethane. Our best estimates for $\Delta H^\circ_{f,298K}$ are: -30.00n-butane, -34.84 n-pentane, -39.84 n-hexane, -44.74 n-heptane, -49.71 n-octane,-32.01 isobutane, -36.49 isopentane, -39.69 neopentane, -41.42 isohexane,-40.72 3-methylpentane, -42.08 diisopropyl, -43.77 neohexane, -46.43isoheptane, -48.84 neoheptane, -53.29 isooctane, and -53.68 hexamethylethane.These are in excellent agreement (typically better than 1 kJ/mol) with theexperimental heats of formation at 298 K obtained from the CCCBDB and/or NISTChemistry WebBook databases. However, at 0 K a large discrepancy between theoryand experiment (1.1 kcal/mol) is observed for only neopentane. This deviationis mainly due to the erroneous heat content function for neopentane used incalculating the 0 K CCCBDB value. The thermochemistry of these systems,especially of the larger alkanes, is an extremely difficult test for densityfunctional methods. A posteriori corrections for dispersion are essential.Particularly for the atomization energies, the B2GP-PLYP and B2K-PLYPdouble-hybrids, and the PW6B95 hybrid-meta GGA clearly outperform other DFTfunctionals.

the journal of physical chemistry. a/the journal of physical chemistry. a.

Benchmark thermochemistry of the C_nH_{2n+2} alkane isomers (n=2--8) and  performance of DFT and composite ab initio methods for dispersion-driven  isomeric equilibria