ON THE ADDITIVITY OF BASIS SET EFFECTS IN SOME SIMPLE FLUORINE CONTAINING SYSTEMS

The reactions F + H2 --> HF + H, HF --> H + F, F --> F+ + e- and F + e- --> F- were used as simple test cases to assess the additivity of basis set effects on reaction energetics computed at the MP4 level. The 6-31G and 6-311G basis sets were augmented with 1, 2, and 3 sets of polarization functions, higher angular momentum polarization functions, and diffuse functions (27 basis sets from 6-31G(d, p) to 6-31 ++ G(3df, 3pd) and likewise for the 6-311G series). For both series substantial nonadditivity was found between diffuse functions on the heavy atom and multiple polarization functions (e.g., 6-31 + G(3d, 3p) vs. 6-31 + G(d, p) and 6-31G(3d, 3p)). For the 6-311G series there is an extra nonadditivity between d functions on hydrogen and multiple polarization functions. Provided that these interactions are taken into account, the remaining basis set effects are additive to within +/- 0.5 kcal/mol for the reactions considered. Large basis set MP4 calculations can also be estimated to within +/- 0.5 kcal/mol using MP2 calculations, est. E(MP4)(6-31 ++ G(3df, 3pd)) almost-equal-to E(MP4)(6-31G(d, p)) + E(MP2)(6-31 ++ G(3df, 3pd)) - E(MP2)(6-31G(d, p)) or E(MP4)(6-31 + G(d, p) + E(MP2)(6-31 ++ G(3df, 3pd)) - E(MP2)(6-31 + G(d, p)) and likewise for the 6-311G series.