Since NSCD can be considered as magnetic circular dichroism induced by the nuclear spin, the mathematical expressions for its fundamental molecular descriptors can be derived along the lines of the derivation of conventional circular dichroism induced by an external (static) magnetic field. Buckingham and Stephens were first to obtain the sum-over-states (SOS) expressions for the Faraday A, B, and C terms of MCD. For closed-shell molecular systems with no degenerate ground or excited states, only the B term is of relevance. Early attempts to compute the MCD terms were based on such SOS expressions. It was later shown that the B term can be obtained from the first-order residues of a quadratic response function involving the electric dipole operator (μ) and, in a non-relativistic context, the orbital Zeeman interaction operator (relative to a common origin O), ĥOZ = e 2me i liO ( liO being the angular momentum operator relative to the origin O). For a given electronic transition from state 0 to state f with an excitation energy of frequency ω f , this is expressed as follows: