The orbital magnetic moment due to rotation or pseudorotation in a molecule or a solid and the corresponding rotational g factor are formulated using the Berry-phase technique and standard density functional plane wave methods. Among the simplest molecules, H-2(+), H-2, C2H2, CH4, and CF4, with known rotational g factors, are used as test cases with excellent results. Alternative, faster localized orbital calculations including the magnetic coupling through heuristic Peierls phase factors are also tested and found to be viable, though less accurate. Application to pseudorotations is exemplified in benzene. It is proposed that these methods will be suited for application to pseudorotations in solids.
