Abstract‘Local’ and ‘nonlocal’ contributions to σd are calculated by INDO molecular orbital methods. It is demonstrated, by means of Pople's approach5 using ‘gauge dependent’ atomic orbitals, that σ is effectively constant for C, N, O or F in a number of chemically different environments and that σ is negligible. The use of Flygare's method12 produces a significant and variable ‘nonlocal’ contribution, which renders σd in this formulation comparable to the gauge dependent term in Ramsey's expression.