  The electric dipole moment of an electron (eEDM) is one of the sensitive
probes of physics beyond the standard model. The possible existence of the eEDM
gives rise to an experimentally observed energy shift, which is proportional to
the effective electric field (Eeff) of a target molecule. Hence, an analysis of
the quantities that enhance Eeff is necessary to identify suitable molecules
for eEDM searches. In the context of such searches, it is generally believed
that a molecule with larger electric polarization also has a larger value of
Eeff. However, our Dirac-Fock and relativistic coupled-cluster singles and
doubles calculations show that the hydrides of Yb and Hg have larger Eeff than
those of fluorides, even though their polarizations are smaller. This is due to
significant mixing of valence s and p orbitals of the heavy atom in the
molecules. This mixing has been attributed to the energy differences of the
valence atomic orbitals and the overlap of the two atomic orbitals based on the
orbital interaction theory.
