Aromaticity and antiaromaticity of Li_xAl₄ clusters: Ring current patterns versus electron counting

Maps of magnetic-field induced current density are computed for a series of lithium-aluminium clusters based on the planar Al₄ cycle: formal 2π systems LiAl₄^- (C_4v), Li ₂Al₄ (D_4h, C_s), and formal 4π systems Li₃Al₄^- (C_s), and Li ₄Al₄ (C_2h). All four species sustain a diatropic σ ring current in the Al₄ cycle. In the 2π systems, although the 4n + 2 π electron count suggests aromaticity and hence diatropicity, the π orbital is magnetically inactive, as in the Al ₄^2- dianion. However, in the 4π formally antiaromatic systems, the π-like HOMO supports an additional paratropic current. Considerations of orbital symmetry and energy, but not electron counting alone, rationalise both computed currents. All calculations were carried out at the coupled Hartree-Fock level in a 6-31G** basis using the CTOCD-DZ (continuous transformation of origin of current density - diamagnetic zero), or ipsocentric, formulation of magnetic response, where current density at any point is obtained with that point itself chosen as the origin of vector potential.