Analysis of non-covalent interactions in (bio)organic molecules using orbital-partitioned localized MP2

Localized molecular orbitals (LMO) are used as basis for an MP2 treatment (LMP2) of electron correlation energies. The major aim is an improved understanding of the non-covalent interactions in large molecules with an emphasis on intra-molecular dispersion effects. A partitioning of the inter-fragment electron correlation energy into electron pairs of different orbital type (i.e., sigma, pi, lone-pairs) is presented. The benzene dimer, 1,4-diphenylbutane conformations, and the tyrosine-glycine dipeptide are used as model systems. For the benzene dimer, comparisons with CCSD(T) data are made in order to analyse the MP2 problems for pi-pi stacking. A comparison of phenyl-phenyl interactions in the benzene dimer and for 1,4-diphenylbutane conformations reveals a very good transferability of dispersion-type contributions to binding from an inter-molecular to an intra-molecular situation. In both systems, the relative (percentage) contributions of sigma-sigma, sigma-pi, and pi-pi pairs to the total inter-fragment correlation energy is a clear signature for the binding mode (pi-stacked vs. T-shaped). For various benzene dimer conformations, we find a linear relation between the MP2 interaction energy error and the correlation contribution from pi-pi pairs. In the dipeptide, also dispersion-type electron correlations between the glycyl amino acid residue and the phenol group are most relevant for folding. This convincingly explains problems of DFT with such systems reported previously. Although in this case only one aromatic ring (and a glycyl moiety) is involved, the same sigma-sigma, sigma-pi, and pi-pi correlations seem to dominate the shape of the potential energy surface.