Description

The Field keyword requests that a finite field be added to a calculation. In Gaussian, 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; care must be taken regarding 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.

Options

#### Read

Reads the coefficients of 34 electric multipole components from the input stream in free format.

#### OldRead

Reads the coefficients of 35 electric multipole components from the input stream, in the old style format (including the monopole term): using format 3D20.10 (the first component is a charge).

#### RWF

Takes the 35 multipole components from the read-write file.

#### ERWF

Extracts only the three electric dipole field components from the read-write file.

#### Checkpoint

Reads the 35 multipole components from the checkpoint file. Chk is a synonym for Checkpoint. Checkpoint is the default with Geom=Check.

#### EChk

Extracts only the three electric dipole field components from the checkpoint file.

#### NoChk

Do not retrieve external field coefficients from the checkpoint file when using Geom=AllCheck.

Limitations

Note that if symmetry is left on during a GVB calculation, the finite field may not lead to correct numerical derivatives if the selected field breaks molecular symmetry. To be safe, use Guess=NoSymm whenever using Field with GVB.

In general, if the electric field causes the wavefunction to have different symmetry than the original molecule, incorrect numerical derivatives can result. Accordingly, you might want to use NoSymm when doing numerical derivatives with Field.

Examples

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

Last updated on: 24 July 2019. [G16 Rev. C.01]