7.5 Mixed Basis Sets
In addition to defining a custom basis set, it is also possible to specify different standard basis sets for different atoms. For example, in a large alkene molecule the hydrogen atoms could be modeled by the STO-3G basis, while the carbon atoms have the larger 6-31G(d) basis. This can be specified within the $basis block using the more familiar basis set labels.
Note: (1) It is not possible to augment a standard basis set in this way; the whole basis needs to be inserted as for a user-defined basis (angular momentum, exponents, contraction coefficients) and additional functions added. Standard basis set exponents and coefficients can be easily obtained by setting the PRINT_GENERAL_BASIS $rem variable to TRUE.
(2) The PURECART flag must be set for all general basis input containing 
angular momentum or higher functions, regardless of whether standard basis sets are entered in this non-standard manner.
The user can also specify different basis sets for atoms of the same type, but in different parts of the molecule. This allows a larger basis set to be used for the active region of a system, and a smaller basis set to be used in the less important regions. To enable this the BASIS keyword must be set to MIXED and a $basis section included in the input deck that gives a complete specification of the basis sets to be used. The format is exactly the same as for the user-defined basis, except that the atom number (as ordered in the $molecule section) must be specified in the field after the atomic symbol. A basis set must be specified for every atom in the input, even if the same basis set is to be used for all atoms of a particular element. Custom basis sets can be entered, and the shorthand labeling of basis sets is also supported.
The use of different basis sets for a particular element means the global potential energy surface is no longer unique. The user should exercise caution when using this feature of mixed basis sets, especially during geometry optimizations and transition state searches.
Example 7.197 Example of adding a user defined non-standard basis set. The user is able to specify different standard basis sets for different atoms.
$molecule
0 1
O
H O oh
H O oh 2 hoh
oh = 1.2
hoh = 110.0
$end
$rem
EXCHANGE hf
BASIS General user-defined general basis
PURECART 2 Cartesian D functions
BASIS2 sto-3g use STO-3G as initial guess
$end
$basis
H 0
6-31G
****
O 0
6-311G(d)
****
$end
Example 7.198 Example of using a mixed basis set for methanol. The user is able to specify different standard basis sets for some atoms and supply user-defined exponents and contraction coefficients for others. This might be particularly useful in cases where the user has constructed exponents and contraction coefficients for atoms not defined in a standard basis set so that only the non-defined atoms need have the exponents and contraction coefficients entered. Note that a basis set has to be specified for every atom in the molecule, even if the same basis is to be used on an atom type. Note also that the dummy atom is not counted.
$molecule
0 1
C
O C rco
H1 C rch1 O h1co
x C 1.0 O xcol h1 180.0
H2 C rch2 x h2cx h1 90.0
H3 C rch2 x h2cx h1 -90.0
H4 O roh C hoc h1 180.0
rco = 1.421
rch1 = 1.094
rch2 = 1.094
roh = 0.963
h1co = 107.2
xco = 129.9
h2cx = 54.25
hoc = 108.0
$end
$rem
EXCHANGE hf
BASIS mixed user-defined mixed basis
$end
$basis
C 1
3-21G
****
O 2
S 3 1.00
3.22037000E+02 5.92394000E-02
4.84308000E+01 3.51500000E-01
1.04206000E+01 7.07658000E-01
SP 2 1.00
7.40294000E+00 -4.04453000E-01 2.44586000E-01
1.57620000E+00 1.22156000E+00 8.53955000E-01
SP 1 1.00
3.73684000E-01 1.00000000E+00 1.00000000E+00
SP 1 1.00
8.45000000E-02 1.00000000E+00 1.00000000E+00
****
H 3
6-31(+,+)G(d,p)
****
H 4
sto-3g
****
H 5
sto-3g
****
H 6
sto-3g
****
$end