  Phenomenological orbital-polarization (OP) terms have been repeatedly
introduced in the single-particle equations of spin-density-functional theory,
in order to improve the description of orbital magnetic moments in systems
containing transition metal ions. Here we show that these ad hoc corrections
can be interpreted as approximations to the exchange-correlation vector
potential A_xc of current-density-functional theory (CDFT). This connection
provides additional information on both approaches: Phenomenological OP terms
are connected to first-principles theory, leading to a rationale for their
empirical success and a reassessment of their limitations and the
approximations made in their derivation. Conversely, the connection of OP terms
with CDFT leads to a set of simple approximations to the CDFT potential A_xc,
with a number of desirable features that are absent from electron-gas-based
functionals.
