Frequency shifts of the Ag  I  4d¹⁰5s  ²S_1∕2(F=0,M_F=0) to 4d⁹5s²  ²D_5∕2(F^′=2,M_F^′=0) electric-quadrupole transition at 330.6  nm due to external fields are calculated using multiconfigurational self-consistent field methods. As this forbidden transition is free from first order Doppler and Zeeman effects, it is under investigation for the realization of an atomic optical clock. The calculated perturbations are the light shift, the blackbody frequency shift, and the quadratic Zeeman shift. Results show that a total uncertainty of 10^−18 could be reach without confining the atoms in a Lamb-Dicke regime in an optical lattice.