The heat of atomization of sulfur trioxide, SO3 — a benchmark for computational thermochemistry

Calibration ab initio (direct coupled cluster) calculations including basis set extrapolation, relativistic effects, inner-shell correlation, and an anharmonic zero-point energy, predict the total atomization energy at 0 K of SO3 to be 335.96 (observed 335.92±0.19) kcal/mol. Inner polarization functions make very large (40 kcal/mol with spd, 10 kcal/mol with spdfg basis sets) contributions to the SCF part of the binding energy. The molecule presents an unusual hurdle for less computationally intensive theoretical thermochemistry methods and is proposed as a benchmark for them. A slight modification of Weizmann-1 (W1) theory is proposed that appears to significantly improve performance for second-row compounds.