The Field keyword requests that a finite field be added to a calculation. In Gaussian 09, 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.
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.
NoChK
Prevents the reading of the field from the checkpoint file.
EChk
Extracts only the three electric dipole field components from the checkpoint file.
Single-point energy, geometry optimizations, frequencies, and Force and Scan calculations.
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.
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
