The need for additional diffuse functions in calculations on small anions: the G2(DD) approach

It is demonstrated that standard basis sets including a single set of diffuse functions are not adequate for calculations on small anions. A new set of diffuse sp-functions is presented and in combination with a simple additivity scheme, they provide very accurate ΔHacid values for the first- and second-row non-metal hydrides. A mean absolute deviation of ca. 2 kJ/mol is observed with the greatest error being 4 kJ/mol.