These keywords indicate that additional basis functions are to be added to the basis set or density fitting basis set specified in the route section for the calculation (respectively). These basis functions appear in a separate section in the input stream, using any of the valid formats (which are described in detail in the discussion of the Gen keyword).
ExtraBasis is most useful for supplying basis functions for elements undefined in a standard basis set. It cannot be used to replace a definition within a built-in basis set, and attempting to do so will result in an error. All basis functions specified with this keyword are added to the ones in the basis set specified in the route section. For these reasons, Gen is often easier to use than ExtraBasis; consult the description for that keyword before deciding to use this one.
ExtraDensityBasis is ignored if no density fitting basis is specified in the route.
The following job uses the 6-31G(d,p) basis set along with an additional diffuse function on all of the carbon atoms:
# HF/6-31G(d,p) ExtraBasis …
SP 1 1.00
0.4380000000D-01 0.1000000000D+01 0.1000000000D+01
The following job supplies additional functions for both the basis set and for density fitting:
#p rblyp/6-31g*/dga1 extrabasis extradensitybasis 6d
HCl using the internally stored 6-31g* AO basis & DGA1 fitting set,
adding f functions to the AO basis, and f & g fitting functions
! here are some extra AO polarization functions
cl 0 Additional basis set functions
F 1 1.00 0.000000000000
p 1 1.00 0.000000000000
! here are some extra fitting functions.
cl 0 Additional density fitting functions
The following job reads in extra basis set data from an external file:
# B3LYP/6-31++G(d) ExtraBasis
B3LYP/6-31++G(d) with extra diffuse functions