Molecular theory of linear magnetoelectricity

We present a theory of molecular contributions to magnetoelectric tensors for anisotropic linear media, due to induced moments at electric quadrupole-magnetic dipole order. This includes a constraint-breaking pseudoscalar (axion) X, evaluated relative to crystallographic axes. In general, the crystal point groups are classified according to the structures they permit for these tensors, including whether non-zero X (and hence violation of the Post constraint) is allowed. The calculated macroscopic observable X is origin independent (dependent) if the time-odd electric dipole-electric dipole polarizability density α¯ij′=0(≠0); a feature that prompts an analogy with the Buckingham effect.