Structure and binding energies of the porphine dimer

Density functional theory optimizations with GGA functionals that include an empirical correction term for the dispersion energy (denoted as -D) have been performed for the van der Waals dimer of porphine. Interaction energies also have been obtained with a recently developed double-hybrid functional (B2-PLYP-D) and at the SCS-MP2 level of theory. The preferred conformer is a parallel displaced complex (1.72 Å lateral displacement) with an interplanar distance of 3.27 Å. The orientation (rotation) of the two monomers has only minor effects (<1 kcal mol− 1) on the binding energy ΔE, implying a certain flexibility of mutual movement around the monomer principal axes. The best estimate for ΔE is − 25 kcal mol− 1 with B2-PLYP-D. The also investigated T-shaped structures are much higher (about 15 kcal mol− 1) in energy. While dispersion contributions are absolutely essential for the binding of all investigated structures, the electrostatic contributions mainly determine the preferred conformations (e.g. displacement versus rotation).