A fully ab initio quartic force field of spectroscopic quality for SO_3

摘要

The quartic force field of SO_3 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 vibration fundamentals are reproduced to within 0.0003 A 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.