A fully ab initio quartic force field of spectroscopic quality for SO3

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 Angstrom and (on average) 1.15 cm(-1), respectively. We recommend the following revised values for the harmonic frequencies: omega(1) = 1082.7, omega(2) = 502.6, omega(3) = 1415.4, omega(4) = 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. (C) 1999 Elsevier Science B.V. All rights reserved.