Orbital contributions to magnetically induced current densities using gauge-including atomic orbitals

We have developed a method to calculate orbital contributions to magnetically induced current density (MICD) susceptibilities in molecules using gauge-including atomic orbitals (GIAO). The methods implemented in the GIMIC program have been used for analyzing orbital contributions to magnetically induced ring-current (MIRC) strengths. We have studied five aromatic, one nonaromatic, and four antiaromatic molecules. We show here that the contributions to the MIRC strength of all orbitals belonging to a given irreducible representation of the molecular point group in the presence of an external magnetic field are divergence free, whereas the MICD susceptibility of the individual orbitals are generally not divergence free. The largest contribution to the MIRC strength of antiaromatic molecules originates from the transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), whereas aromatic molecules have significant contributions involving many occupied orbitals. The MIRC contributions of σ orbitals are significant for planar molecules with a strained molecular structure.