Basis sets incorporating diffuse functions

Notice that we want to use 2 d-type functions in the basis. This is because for correlated wave functions the d-type functions serve two purposes, which cannot easily be accomplished by one basis function First they serve as polarization functions, polarizing the electron density in the bonding region, a feature that is especially important in 7t-bonded systems. Secondly, they give important contributions to the angular correlation effects, which is very important in linear molecules. Studies of the ANO s shows that the two properties of the d-type functions cannot be incorporated into one function. Hie polarization d is rather diffuse, while the correlating d is a much more contracted function. We therefore prefer to include two d-type functions in the basis set... 

The first two rows of Table 1 make readily apparent the poor suitability of both ST0-3G and 3-21G for H-bonds the intermolecular R(00) distance is some 0.2 A too short. The 6-31G(d) set (also known as 6-31G" ) contains a better representation of the inner shells as well as polarization functions on O. The SCF distance of 2.971 A computed with this basis set is a substantial improvement over those from the smaller basis sets, STO-3G and 3-21G. Further improvements in the basis set make relatively minor changes in this distance. For example, addition of p-functions to H elongates the bond by 0.009 A, whereas augmentation of O by -f- functions (a diffuse sp-shell) reduces the distance by 0.007 A. It is instructive to note that the latter two effects are not additive. That is, one might naively expect a net bond elongation of 0.002 A to result from incorporation of both H p-functions and O -F functions by addition of the two latter effects. However, the 6-31 -F G(d,p) R (OO) distance reported in Table 1 is 2.988 A, 0.017 A longer than the 6-31G(d) value. 

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