Ab initio total atomization energies of small molecules - Towards the basis set limit

Total atomization energies (TAEs) of about a dozen small polyatomic molecules have been calculated at the CCSD(T) level using correlation consistent basis sets of up to [7s6p5d4f3g2h] quality, and including core correlation. Single bond energies are close to convergence with such basis sets: for multiple bonds, extrapolation remains mandatory. 'Augmented' basis sets specifically designed for anions yield improved atomization energies for highly polar molecules. An extrapolation of the form A + B/(l + 1/2)(4) + C/(l + 1/2)(6), with l the maximum angular momentum in the basis set, is found to yield TAEs accurate, on average, to 0.5 kcal/mol using at most [spdfgh] basis sets. Using [spdfgh] basis sets and a small additivity correction for triple bonds, this can be reduced to 0.2 kcal/mol.