Physical origins of the stability of aromatic amino acid core ring-polycyclic hydrocarbon complexes: a post-Hartree-Fock and density functional study.

ABSTRACT

Noncovalent interactions between four aromatic amino acid core rings and polycyclic hydrocarbons (up to 58 carbon atoms) were investigated at the post-Hartree-Fock level of theory and with the aid of the density functional theory. In particular, the intermolecular interaction energy was partitioned into physically meaningful contributions using the hybrid variational-perturbational decomposition scheme. It was found that electrostatic interactions are completely quenched by associated exchange repulsion. In terms of magnitude, the latter component is 2-3 times larger than the former one. As a result, the complexes are solely stabilized by attractive dispersion forces. Moreover, the SAPT-DFT framework was employed with an eye toward a comparison of various approaches used to compute the dispersion energy. As part of a model study, the impact of the correct representation of the electron density on the description of intermolecular interactions was analyzed as well. It was shown that in the case of the investigated complexes, it is sufficient to augment the basis set with diffuse functions only in the region of the mutual overlap of molecules in stacked alignment.