  Frequency shifts of the ^199Hg^+ 5d^10 6s ^2S_1/2 (F=0, M_F=0) to 5d^9 6s^2
^2D_5/2 (F=2, M_F=0) electric-quadrupole transition at 282 nm due to external
fields are calculated, based on a combination of measured atomic parameters and
ab initio calculations. This transition is under investigation as an optical
frequency standard. The perturbations calculated are the quadratic Zeeman
shift, the scalar and tensor quadratic Stark shifts, and the interaction
between an external electric field gradient and the atomic quadrupole moment.
The quadrupole shift is likely to be the most difficult to evaluate in a
frequency standard and may have a magnitude of about 1 Hz for a single ion in a
Paul trap.
