Benchmark quality total atomization energies of small polyatomic molecules

Successive coupled-cluster [CCSD(T)] calculations in basis sets of spdf, spdfg, and spdfgh quality, combined with separate Schwartz-type extrapolations A + B/(l + 1/2)(alpha) of the self-consistent field (SCF) and correlation energies, permit the calculations of molecular total atomization energies (TAEs) with a mean absolute error of as low as 0.12 kcal/mol. For the largest molecule treated, C2H4, we find Sigma D-0=532.0 kcal/mol, in perfect agreement with experiment. The aug'-cc-pV5Z basis set recovers on average about 99% of the valence correlation contribution to the TAE, and essentially the entire SCF contribution. (C) 1997 American Institute of Physics.