  According to the Schiff theorem nuclear electric dipole moment (EDM) is
completely shielded in a neutral atom by electrons. This makes a static nuclear
electric dipole moment (EDM) unobservable. Interaction with the axion dark
matter field generates nuclear EDM $d=d_0 \cos (\omega t)$ oscillating with the
frequency $\omega= m_a c^2/\hbar$ . This EDM generates atomic EDM proportional
to $\omega^2$. This effect is strongly enhanced in molecules since nuclei move
slowly and do not produce as efficient screening of oscillating nuclear EDM as
electrons do. An additional strong enhancement comes due to a small energy
interval between rotational molecular levels. Finally, if the nuclear EDM
oscillation frequency is in resonance with a molecular transition, there may be
a significant resonance enhancement.
