An overview of applications of the recently developed gauge including magnetically induced current method (GIMIC) is presented. The GIMIC method is used to obtain magnetically induced current densities in molecules. It provides detailed information about electron delocalization, aromatic character, and current pathways in molecules. The method has been employed in aromaticity studies on hydrocarbons, complex multi-ring organic nanorings, Möbius twisted molecules, inorganic and all-metal molecular rings and open-shell species. Recent studies on hydrogen-bonded molecules indicate that GIMIC can also be used to estimate hydrogen-bond strengths without fragmentation of the system. Preliminary results are presented on the applicability of GIMIC for investigating current transport in molecules attached to clusters simulating molecular conductivity measurements. Advantages and limitations of the GIMIC method are reviewed and discussed.

The magnetically induced current densities for ring-shaped hydrocarbons are studied at the density functional theory (DFT) and second-order Møller-Plesset (MP2) levels using gauge-including atomic orbitals. The current densities are calculated using the gauge-including magnetically induced current approach. The calculations show that all studied hydrocarbon rings sustain strong diatropic and paratropic ring currents when exposed to an external magnetic field, regardless whether they are unsaturated or not. For nonaromatic rings, the strength of the paratropic current flowing inside the ring is as large as the diatropic one circling outside it, yielding a vanishing net ring current. For aromatic molecules, the diatropic current on the outside of the ring is much stronger than the paratropic one inside, giving rise to the net diatropic ring current that is typical for aromatic molecules. For antiaromatic molecules, the paratropic ring-current contribution inside the ring dominates. For homoaromatic molecules, the diatropic current circles at the periphery of the ring. The ring current is split at the CH(2) moiety; the main fraction of the current flow passes outside the CH(2) at the hydrogens, and some current flows inside the carbon atom. The diatropic current does not take the through-space short-cut pathway, whereas the paratropic current does take that route. Calculations of the ring-current profile show that the ring current of benzene is not transported by the pi electrons on both sides of the molecular ring. The strongest diatropic ring current flows on the outside of the ring and in the ring plane. A weaker paratropic current circles inside the ring with the largest current density in the ring plane. Due to the ring strain, small unconjugated and saturated hydrocarbon rings sustain a strong ring current which could be called ring-strain current. Nuclear magnetic shieldings calculated for 1,3,5-cycloheptatriene and homotropylium at the DFT and MP2 levels agree well with experimental values.

The gauge-including magnetically induced current method for calculating the components of the current-density tensor using gauge-including atomic orbitals has been extended to treating open-shell molecules. The applicability of the method is demonstrated by calculations of first-order induced current densities on cyclobutadiene, Al(3), and B(3) at correlated ab initio levels of theory. For comparison, current-density calculations were also performed on the lowest closed-shell singlet state of cyclobutadiene as well on the closed-shell Al(3)(-) and B(3)(-) anions. The ring-current susceptibilities of the open-shell species are computed at the Hartree-Fock self-consistent-field, second-order Møller-Plesset perturbation theory, and coupled-cluster singles and doubles levels, whereas for the closed-shell systems also density functional theory calculations were employed. Explicit values for the current strengths caused by α and β electrons as well as the difference, representing the spin current, were obtained by numerical integration of the current-density contributions passing a plane perpendicular to the molecular ring. Comparisons of the present results to those recently obtained for the lowest triplet state of biphenyl emphasize that electron correlation effects must be considered for obtaining an accurate description of spin-current densities.

The aromatic pathways and the degree of aromaticity of expanded porphyrins have been determined by explicit calculations of the routes and strengths of the magnetically induced currents using the gauge-including magnetically induced current (GIMIC) approach. Density functional theory calculations show that the doubly twisted hexaphyrins fulfilling Hückel's (4n + 2) pi-electron rule for aromaticity and those obeying the 4n pi-electron rule for antiaromaticity are aromatic and antiaromatic, respectively. The investigated [26]hexaphyrin (2) and (3) and [30]hexaphyrin (5) isomers are aromatic, and [28]hexaphyrin (4) is antiaromatic. The formally antiaromatic [24]hexaphyrin (1) does not sustain any strong ring current and must be considered nonaromatic. A detailed analysis of the current pathways of the hexaphyrins is presented. It was found that the current pathways of the investigated aromatic hexaphyrins are not always dominated by the flow along the inner route through the non-hydrogenated C-N-C moieties, as previously proposed. The current flow is often split into two branches at the pyrrole rings, but sometimes it takes the outer route via the C=C bond of the pyrrole. The current pathway of the weak paratropic ring current of [24]hexaphyrin is dominated by the outer C=C route. The calculations show that the routes of the current transport cannot be assessed merely by inspection or from nucleus independent chemical shifts; explicit calculations of the current pathways are compulsory. The current-density studies also show that the pyrrole rings do not sustain any strong ring currents of their own.