“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. ”
"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). "