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The correlation consistent composite approach (ccCA) has been used to compute the enthalpies of formation (ΔH(f)'s) for 60 closed-shell, neutral hydrocarbon molecules selected from an established set (Cioslowski et al., J. Chem. Phys. 2000, 113, 9377). This set of thermodynamic values includes ΔH(f)'s for hydrocarbons that span a range of molecular sizes, degrees of aromaticity, and geometrical configurations, and, as such, provides a rigorous assessment of ccCA's applicability to a variety of hydrocarbons. The ΔH(f)'s were calculated via atomization energies, isodesmic reactions, and hypohomodesmotic reactions. In addition, for 12 of the aromatic molecules in the set that are larger than benzene, the energies of ring-conserved isodesmic reactions were used to calculate the ΔH(f)'s. Using an atomization energy approach to determine the ΔH(f)'s, the lowest mean absolute deviation (MAD) from experiment achieved by ccCA for the 60 hydrocarbons was 1.10 kcal mol(-1). The use of the mixed Gaussian/inverse exponential complete basis set extrapolation scheme (ccCA-P) in conjunction with hypohomodesmotic reaction energies resulted in a MAD of 0.87 kcal mol(-1). This value is compared with MADs of 1.17, 1.18, and 1.28 kcal mol(-1) obtained via the Gaussian-4 (G4), Gaussian-3 (G3), and Gaussian-3(MP2) [G3(MP2)] methods, respectively (using the hypohomodesmotic reactions).

Systematic truncation of the correlation consistent basis sets has been investigated in first and second row atoms and molecules to better understand basis set requirements for density functional theory, particularly the need for high angular momentum functions, as well as to understand possible computational cost savings that could be achieved by using reduced basis sets. The truncation scheme employed follows that recently introduced for ab initio methods [B. Mintz et al., J. Chem. Phys. 121, 5629 (2004)]. Properties examined in the current study include geometries, ionization potentials, electron affinities, and dissociation energies. In general, this investigation shows that a degree of truncation of higher angular momentum functions is possible with limited impact upon energetic properties, and does result in useful CPU time savings. However, not all properties investigated have the same level of dependence upon high angular momentum functions, and, thus, careful selection of truncated basis sets should be made.

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A computational study of diatomic NiAl is reported. Molecular properties evaluated include the equilibrium bond length (r e ), equilibrium stretching frequency (x e ), doublet-quartet energy splitting, and nickel-aluminum bond strength. Several interesting conclusions have resulted from this research. First, convergence in calculated properties is smoother with recently reported correlation consistent basis sets than earlier basis sets for Ni and Al. Second, with the exception of bond strength, basis set limit properties extrapolated using correlation basis sets are in agreement with reported data. Third, this research suggests that caution may be needed with regard to the use of DFT for developing interatomic potentials for larger scale simulations. For example, B97-1 showed better agreement with reported r e for 2 NiAl than B3LYP. However, the situation was reversed for the calculation of x e . With respect to bond strength, the situation is unclear due to the scatter among experiment and calculations.

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