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An inexpensive method for accurate simulation of π contributions to ring currents is proposed. A conjugated carbon framework is formally replaced by a set of hydrogen atoms bearing single 1s (STO-3G) orbitals. Calculation, with the ipsocentric CTOCD-DZ method, of the in-plane σ current density induced by a perpendicular magnetic field then gives a close numerical match to the out-of-plane π current density of the original carbon system at a height of 1 bohr. This ‘pseudo-π’ method depends on the one-to-one correspondence in symmetry between σ and π orbitals and is capable of reproducing the general pattern of π current, its specific features and its orbital decomposition for polycyclic hydrocarbon systems. [Copyright &y& Elsevier]

Localised-orbital analysis of the current density induced by a perpendicular magnetic field in square-planar Al42− demonstrates the intrinsic non-localisability of the σ electrons of this metallic cluster and confirms their dominant role in its diamagnetic ring current. Though delocalised, the π electrons do not give rise to a significant ring current. [Copyright &y& Elsevier]