Gaussian 03 Online Manual
The Field keyword requests that a finite field be added to calculation. In Gaussian 03, the field can either involve electric multipoles (through hexadecapoles) or a Fermi contact term. Field requires a parameter in one of these two formats:
M±N or F(M)N
where M designates a multipole, and F(M) designates a Fermi contact perturbation for atom M (following the ordering in the molecule specification section of the input file). N*0.0001 specifies the magnitude of the field in atomic units in the first format, and specifies the magnitude of the Fermi contact perturbation in the second format.
Thus, Field=X+10 applies an electric dipole field in the X direction of 0.001 au, while Field=XXYZ-20 applies the indicated hexadecapole field with magnitude 0.0020 au and direction opposite to the default (which is determined by the standard orientation). Similarly, Field=F(3)27 applies a perturbation of 0.0027 times the spin density on atom 3.
Note that the coefficients are those of the Cartesian operator matrices; be careful of the choice of sign convention when interpreting the results.
All parameters are in the input orientation.
The field specification parameter may be placed among any other options as desired. Archiving is disabled when Field is specified.
Note that if symmetry is left on during a GVB calculation, the finite field will or will not lead to correct numerical derivatives, depending on whether the selected field breaks molecular symmetry. To be safe, use Guess=NoSymm whenever using Field with GVB.
To perform geometry optimizations in the presence of an electric field, you must use Opt=Z-Matrix NoSymm keywords and define the input geometry either in traditional Z-matrix coordinates or symbolic Cartesian coordinates. Here is an example using a Z-matrix:
# RHF/3-21G Field=x+60 Opt=Z-Matrix NoSymm Z-Matrix optimization 0 1 C H 1 B1 H 1 B2 2 A1 H 1 B3 2 A2 3 D1 H 1 B4 2 A3 3 D2 B1 1.070000 B2 1.070000 B3 1.070000 B4 1.070000 A1 109.471203 A2 109.471203 A3 109.471231 D1 120.000015 D2 -119.999993
Here is an example using symbolic Cartesian coordinates:
# HF/6-31G(d) Opt=Z-Matrix Field=z-50 NoSymm Symbolic Cartesian coordinates optimization 0 1 O 0 x1 y1 z1 H 0 x2 y2 z2 H 0 x3 y3 z3 x1=0.0 y1=0.0 z1=0.12 x2=0.0 y2=0.75 z2=-0.46 x3=0.0 y3=-0.75 z3=-0.46