Describing both Dispersion and Electronic Structure Using Density Functional Theory: The Case of Metal-Phthalocyanine Dimers

Dispersion interactions often determine the structure of crystals and thin layers of organic semiconductors. However, treatment of dispersion is outside the reach of (semi)-local approximations to the exchange-correlation (xc) functional and of hybrid functionals based on semi-local correlation. We offer an approach that treats both dispersion and electronic structure within a computationally tractable scheme, without modifying the xc functional. This is based on adding the leading interatomic dispersion term via pair-wise ion-ion interactions to a suitable non-empirical hybrid functional, with dispersion coefficients and van der Waals radii determined from first-principles using the recently proposed ``TS-vdW'' scheme. The approach is demonstrated for the case of weakly bound metal-phthalocyanine dimers. It is additionally compared to the semi-empirical M06 functional. We find that both PBEh+vdW and M06 predict the electronic structure and the equilibrium geometry well, but with significant differences in the binding energy and in their different asymptotic behavior.