quartic force field of spectroscopic quality for SO 3 ∗

The quartic force field of SO3 was computed fully ab initio using coupled cluster (CCSD(T)) methods and basis sets of up to spdfgh quality. The effect of inner-shell correlation was taken into account. The addition of tight d functions is found to be essential for accurate geometries and harmonic frequencies. The equilibrium geometry and vibrational fundamentals are reproduced to within 0.0003 Å and (on average) 1.15 cm−1, respectively. We recommend the following revised values for the harmonic frequencies: ω1=1082.7, ω2=502.6, ω3=1415.4, ω4=534.0 cm −1. In addition, we have shown that the addition of inner polarization functions to second-row elements is highly desirable even with more approximate methods like B3LYP, and greatly improves the quality of computed geometries and harmonic frequencies of second-row compounds at negligible extra computational cost. For larger such molecules, the B3LYP/VTZ+1 level of theory should be a very good compromise between accuracy and computational cost. In memory of my colleague Dr. Jacqueline Libman OBM (1941–1997)