A STUDY OF O-3, S-3, CH-2, AND BE-2 USING KOHN-SHAM THEORY WITH ACCURATE QUADRATURE AND LARGE BASIS-SETS

Density functional calculations with accurate quadrature and large bas is sets are reported on ozone, thiozone, methylene, and the beryllium dimer. The Kohn-Sham self-consistent procedure is used throughout and nonlocal gradient corrected functionals (B-LYP, B-P) are used. The sys tems and properties chosen are notoriously difficult problems for accu rate computation, nevertheless, the density functional methods perform well. It is found, in line with calculations on more straightforward problems, that the functionals involving gradient corrections are more reliable than local density functionals. The B-LYP prediction is with in 5 kcal/mol of the most accurate ab initio data on the C2v-D3h energ y gap in ozone and thiozone. The B-LYP functional performs reasonably well on the vibrational frequencies of ozone whilst the B-P functional gives very accurate results for the structural parameters of ozone. B -LYP is used to estimate the singlet-triplet splitting in methylene wi th the triplet treated in both a restricted and unrestricted formalism . In either case the correct energy gap is reproduced within 2 kcal/mo l. The bond length and low frequency of the beryllium dimer are reprod uced accurately by the gradient corrected functionals but the binding energy is overestimated. However, B-LYP only overestimates the binding energy by 4 kcal/mol, which is an encouraging result given the diffic ulty of the problem. This paper can therefore be seen as further evide nce that density functionals methods, and in particular methods based on nonlocal functionals, can predict molecular properties reasonably a ccurately for systems that have multireference character.