Excited electronic potential-energy surfaces and transition moments for the H3 system

Four electronic states of H3 have been studied using a multiple-reference double-excitation configuration-interaction method with an extensive basis set of 75 Gaussian-type atomic orbitals. A total of 1340 ab initio points were calculated over a wide range of H3 molecular geometries. These four states include the ground state and the Rydberg 2s A12′ and 2pz A22″ states, as well as the state that in equilateral triangular geometry is related to the ground state by a conical intersection. Electric-dipole transition moments were also obtained between these states. The results show that the atomic and diatomic energetic asymptotes are accurately described. The barriers, wells, and energy differences also show good agreement compared to literature values, where available. The potential energies of the ground state and the 2pz A22″ Rydberg state display smooth and regular behavior and were fitted over the whole molecular geometries using a rotated Morse curve-cubic spline approach. The other two potential-energy surfaces reveal more complicated behaviors, such as avoided crossings, and will require a different fitting procedure to obtain global fitting. Finally, dynamical implications of these potential surfaces and electric-dipole transition moments are discussed

Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like

The structure of liquid water at ambient conditions is studied in ab initio molecular dynamics simulations using van der Waals (vdW) density-functional theory, i.e. using the new exchange-correlation functionals optPBE-vdW and vdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highly directional hydrogen-bonds, which are enhanced by standard functionals. This brings about a softening of the microscopic structure of water, as seen from the broadening of angular distribution functions and, in particular, from the much lower and broader first peak in the oxygen-oxygen pair-correlation function (PCF), indicating loss of structure in the outer solvation shells. In combination with softer non-local correlation terms, as in the new parameterization of vdW-DF, inclusion of vdW interactions is shown to shift the balance of resulting structures from open tetrahedral to more close-packed. The resulting O-O PCF shows some resemblance with experiment for high-density water (A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but not directly with experiment for ambient water. However, an O-O PCF consisting of a linear combination of 70% from vdW-DF2 and 30% from experiment on low-density liquid water reproduces near-quantitatively the experimental O-O PCF for ambient water, indicating consistency with a two-liquid model with fluctuations between high- and low-density regions

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

Electronically and sterically induced structural distortions in square-planar d(8) complexes

The solid-state structure of the cationic MeO-Biphep Rh(I) compound [Rh((S)-MeO-Biphep)-(P{OMe}(3))(2))]BF4 (3) has been determined by X-ray diffraction. The four P-donors deviate markedly from square-planar geometry, with the phosphite ligands P2 and P2' ca. +/-0.61(7) Angstrom from the P1-Rh-P1' plane. This distortion resembles that found for PdBr(p-NCC6H4)-((S)-MeO-Biphep) (1). Density functional calculations on a series of systematically varied models of 1 reveal three major components to be responsible for the observed distortion from square-planar geometry: (i) attractive aromatic ring pi-pi interactions, (ii) electronic stabilization of coplanar aromatic rings in pseudo-trans positions, and (iii) P-phenyl and MeO-Biphep-phenyl intraligand repulsive steric interactions. Additional DFT studies on the p-tolyl-Binap analogue of i, PdBr(p-NCC6H4)((R)-p-Tol-Binap) (2), explain the source of the extremely long Pd-PB bond distance, 2.437(1) Angstrom, in 2. Despite the structural similarity between 1 and 2, the calculations rationalize the observation of a pronounced distortion from a square-planar geometry in the former that is essentially absent in the latter