in which the parameter a was empirically chosen to be 1.1 to best balance the graphical representation of various kinds of interactions. Isosurface of IRI=1.0 a.u. is usually adopted in the IRI analysis, but it can be slightly adjusted to obtain a better representation for a specific system. where the dark blue isosurfaces clearly reveal all covalent bonds, including those between the two nonacene molecules. Additionally, the significant steric effect caused by the covalent dimerization, as well as the large-area - stacking, are also visible.