Custom Basis Sets

If the basis set you desire is not already defined in PSI3, a custom set may be used by specifying its exponents and contraction coefficients (either in the input file or another file named basis.dat.) A contracted Cartesian Gaussian-type orbital

$$\phi_{\rm CGTO} = x^ly^mz^n\sum_i^N C_i \exp(-\alpha_i[x^2+y^2+z^2])$$ (1)

where

$$L = l+m+n$$ (2)

is written as

basis: (
  ATOM_NAME: "BASIS_SET_LABEL" = (
    (L (C1  alpha1)
       (C2  alpha2)
       (C3  alpha3)
       ...
       (CN  alpha4))   
    )
  )

One must further specify whether Cartesian or spherical harmonics Gaussians are to be used. One can specify that in two ways:

• It can be done on a basis by basis case, such as

  basis: (
    "BASIS_SET_LABEL1":puream = true
    "BASIS_SET_LABEL2":puream = false
    "BASIS_SET_LABEL3":puream = true
    ....
  )
  

  By default, if puream is not given for a basis, then Cartesian Gaussians will be used.
• The choice between Cartesian or spherical harmonics Gaussian can be made globally by specifying puream keyword in the standard input section, e.g.

  psi: (
    ...
    puream = true
    ...
  )
  

Note that currently PSI3 cannot handle basis sets that consist of a mix of Cartesian and spherical harmonics Gaussians.

Note that the basis set must be given in a separate basis: section of input, outside all other sections (including psi:). For example, the PSI3 DZP basis set for carbon could be specified as:

basis: (
  carbon: "DZP" = (
    (S (   4232.6100      0.002029) 
       (    634.8820      0.015535)
       (    146.0970      0.075411)
       (     42.4974      0.257121)
       (     14.1892      0.596555) 
       (      1.9666      0.242517))
    (S (      5.1477      1.0))
    (S (      0.4962      1.0))
    (S (      0.1533      1.0))
    (P (     18.1557      0.018534)
       (      3.9864      0.115442)
       (      1.1429      0.386206)
       (      0.3594      0.640089))
    (P (      0.1146      1.0))
    (D (      0.75        1.0))
  )
)

Here are a couple of additional points that may be useful when specifying customized basis sets:

• Normally the basis.dat file is placed in the same directory as the main input file, but it may also be placed in a global location specified by the keyword basisfile:

    basisfile = "/home/users/tool/chem/h2o/mybasis.in"
  

• To scale a basis set, a scale factor may be added as the last item in the specification of each contracted Gaussian function. For example, to scale the S functions in a 6-31G** basis for hydrogen, one would use the following

    hydrogen: "6-31G**" =
        ( (S (    18.73113696     0.03349460)
             (     2.82539437     0.23472695)
             (     0.64012169     0.81375733) 1.2 )
          (S (     0.16127776     1.00000000) 1.2 )
          (P (     1.10000000     1.00000000))
         )
  

  In this example, both contracted S functions have their exponents scaled by a factor of (1.2)$^2$ = 1.44. The output file should show the exponents after scaling.