  The observation of an electron electric dipole moment (eEDM) would have major
ramifications for the standard model of physics. Polar molecules offer a
near-ideal laboratory for such searches due to the large effective electric
field (${\bf F}_{\rm eff}$), on order of tens of GV/cm that can be easily
oriented in the lab frame. We present an improved method for simply and
accurately determining ${\bf F}_{\rm eff}$, in a heavy polar molecule, allowing
for a quick determination of candidates for an eEDM experiment. We apply this
method to ThO and ThF$^{\rm +}$, both of which possess metastable $^3\Delta$
electronic states. The values of ${\bf F}_{\rm eff}$ in ThO and ThF$^{\rm +}$
are estimated to be 104 GV/cm and 90 GV/cm respectively, and are therefore two
of the best known candidates for the eEDM search.
