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A theoretical model chemistry designed to achieve high accuracy for enthalpies of formation of atoms and small molecules is described. This approach is entirely independent of experimental data and contains no empirical scaling factors, and includes a treatment of electron correlation up to the full coupled-cluster singles, doubles, triples and quadruples approach. Energies are further augmented by anharmonic zero-point vibrational energies, a scalar relativistic correction, first-order spin-orbit coupling, and the diagonal Born-Oppenheimer correction. The accuracy of the approach is assessed by several means. Enthalpies of formation (at 0 K) calculated for a test suite of 31 atoms and molecules via direct calculation of the corresponding elemental formation reactions are within 1 kJ mol(-1) to experiment in all cases. Given the quite different bonding environments in the product and reactant sides of these reactions, the results strongly indicate that even greater accuracy may be expected in reactions that preserve (either exactly or approximately) the number and types of chemical bonds.

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9568-9575error. The following gives the choices made for the BDEs.CH. The CH BDEs of several molecules have been determined.70 However, Me3Si-CHJ1 (BDE of 99.2 kcal/mol) is the most similar to the present systems. Therefore, the value of 99 kcal/mol is used for all C H BDEs.SiH. The closest analogue to the systems of interest is the H3Si-SiH, molecule, which has a BDE of 86.3 kcal/m~l.~' The value of 86 kcal/mol is used for all SiH BDEs.CeH. The BDE of Ge-H for CH3GeH3 is 83 k c a l / m~l .~~ The BDE for GeH4 is given as 8473 and 8972 kcal/mol by different experimental researchers. Also, a theoretical value of 84.8 kcal/mol has been determined by Binning and C~r t i s s . '~ The BDE for Me3GeH has been determined to be 82 kcal/mol. The value of 82 kcal/mol is chosen for all G e H BDEP except for that of CH3GeH3, since the electronegativity of Si, Ge, and Sn should have similar effects as that of the bulky methyl groups of Me3GeH.SnH. The BDE of SnH4 is 71.6 kcal/m~l.'~ The value for Me3Sn-H76s77 and Bu3Sn-H7* is 74 kcal/mol. Therefore, the value of 74 kcal/mol is used for all Sn-H BDEs. (73) Noble, P. N.; Walsh, R. Inr.Abstract: Correlated level ab initio calculations (large basis set MP2, and MP4, CCSD, and CCSD(T) computations) have been performed for 13 conformers of neutral glycine, including all 8 possible conformers with planar heavy-atom arrangements. These calculations resulted in accurate geometric structures, relative energies, harmonic vibrational frequencies, and infrared intensities for all conformers. The structural results obtained support the rotational constants measured for the two lowest-energy forms of glycine, and their high accuracy should be profitable in the search for other conformers by rotational spectroscopy. Energetic, structural, and quadratic force field results indicate possible model improvements for an existing gas-phase electron-diffraction study of the lowest-energy conformer. Predictions, probably accurate to within about 100 cm-I, are made for the order and relative energy of all conformers considered. (1) (a) Sellers, H. L.; Schifer, L. J. Am. Chem. Soc. 1978, 100, 7728. (b) Schifer, L.; Sellers, H. L.; Lovas, F. J.; Suenram, R. D. J. Am. Chem. SOC. 1980, 102, 6566. (2) Frey, R. F.; Coffin, J.; Newton, S. Q.; Ramek, M.; Cheng, V. K. W.; Momany, F. A.; Schifer, L. J. Am. Chem. SOC. 1992, 124, 5369. (3) Sam, K.; Klimkowski, V. J.; Ewbank, J. D.; Van Alsenoy, C.; Schifer, L.

The recently developed high-accuracy extrapolated ab initio thermochemistry method for theoretical thermochemistry, which is intimately related to other high-precision protocols such as the Weizmann-3 and focal-point approaches, is revisited. Some minor improvements in theoretical rigor are introduced which do not lead to any significant additional computational overhead, but are shown to have a negligible overall effect on the accuracy. In addition, the method is extended to completely treat electron correlation effects up to pentuple excitations. The use of an approximate treatment of quadruple and pentuple excitations is suggested; the former as a pragmatic approximation for standard cases and the latter when extremely high accuracy is required. For a test suite of molecules that have rather precisely known enthalpies of formation {as taken from the active thermochemical tables of Ruscic and co-workers [Lecture Notes in Computer Science, edited by M. Parashar (Springer, Berlin, 2002), Vol. 2536, pp. 25-38; J. Phys. Chem. A 108, 9979 (2004)]}, the largest deviations between theory and experiment are 0.52, -0.70, and 0.51 kJ mol(-1) for the latter three methods, respectively. Some perspective is provided on this level of accuracy, and sources of remaining systematic deficiencies in the approaches are discussed.