π-π stacking tackled with density functional theory

Through comparison with ab initio reference data, we have evaluated the performance of various density functionals for describing π-π interactions as a function of the geometry between two stacked benzenes or benzene analogs, between two stacked DNA bases, and between two stacked Watson–Crick pairs. Our main purpose is to find a robust and computationally efficient density functional to be used specifically and only for describing π-π stacking interactions in DNA and other biological molecules in the framework of our recently developed QM/QM approach "QUILD". In line with previous studies, most standard density functionals recover, at best, only part of the favorable stacking interactions. An exception is the new KT1 functional, which correctly yields bound π-stacked structures. Surprisingly, a similarly good performance is achieved with the computationally very robust and efficient local density approximation (LDA). Furthermore, we show that classical electrostatic interactions determine the shape and depth of the π-π stacking potential energy surface

Theoretical study of the He-HF+ complex. I. The two asymptotically degenerate ground state potential energy surfaces.

Two three-dimensional potential energy surfaces (PESs) are reported for the cationic complex He-HF+; they are degenerate for linear geometries of the complex and correlate with the doubly degenerate X2Pi ground state of the HF+monomer. The PESs are computed from the interaction energies of the neutral dimer and the ionization potentials of the He-HF complex and the HF molecule. Ionization potentials are obtained from the outer valence Green's function (OVGF) method, while the energies of the neutral species are computed by means of the single and double coupled-cluster method with perturbative triples [CCSD(T)]. For comparison, interaction energies of the ionic complex were computed also by the use of the partially spin-restricted variant of the CCSD(T) method. After asymptotic scaling of the OVGF results, good agreement is found between the two methods. A single global minimum is found in the PES, for the linear He-HF+ geometry. The well depth and equilibrium separation are 2.240 A and 1631.3 cm(-1), respectively, at an HF+ bond length r=1.0012 A, in rather good agreement with results of Schmelz and Rosmus [Chem. Phys. Lett. 220, 117 (1994)]. The well depth depends much more strongly on the internuclear H-F separation than in the neutral He-HF complex and the global minimum in a full three-dimensional PES occurs at r=1.0273 A

A theoretical study of 1:1 and 1:2 complexes of acetylene with nitrosyl hydride

Ab initio calculations at MP2 computational level using aug-cc-pVTZ basis set were used to analyze the interactions between 1:1 and 1:2 complexes of acetylene and nitrosyl hydride. The structures obtained have been analyzed with the atoms in molecules and the density functional theory-symmetry adapted perturbation theory methodologies. Four minima were located on the potential energy surface of the 1:1 complex. Twenty-four different structures have been obtained for the 1:2 complexes. Five types of interactions are observed, CH·O, CH·N, NH·π hydrogen bonds and orthogonal interactions between the π clouds of triple bond, or the lone pair of oxygen with the electron-deficient region of the nitrogen atom. Stabilization energies of the 1:1 and 1:2 clusters including basis set superposition error and ZPE are in the range 3-8 and 6-17 kJ mol -1 at MP2/aug-cc-pVTZ computational level, respectively. Blue shift of NH bond upon complex formation in the ranges between 18-30 and 20-96 cm -1 is predicted for 1:1 and 1:2 clusters, respectively. The total nonadditive energy in the 1:2 cluster, calculated as the sum of the supermolecular nonadditive MP2 energy and the three-body dispersion energy, presents values between - 1.48 and 1.20 kJ mol -1. © Springer Science+Business Media, LLC 2011.Peer Reviewe