FIELD Fx Fy Fz
where Fx, Fy and Fz are the x, y and z components of the field, in atomic units. ( a sensible magnitude for a field ia approximately 0.001 a.u.)
You can also add field gradient terms using,
FIELDGRAD Fxx Fxy Fxz Fyy Fyz Fzz
where the arguments are the six components of the electric field gradient in atomic units.
It is also possible to include an external lattice of point charges. The format of the LATTICE command is,
x1 y1 z1 charge1
x2 Y2 z2 charge2
where you have to give the cartesian coordinates (x,y,z) for each lattice point (in atomic units), and its charge. The charge can be positive or negative and can be fractional.
For all of the command in this section you should remember that, if you have specified the molecular SYMMETRY then the presence of an external field or field gradient or lattice may lower that symmetry.