A. Peeters

Atomic charges, dipole moments, and Fukui functions using the Hirshfeld partitioning of the electron density

In the Hirshfeld partitioning of the electron density, the molecular electron density is decomposed in atomic contributions, proportional to the weight of the isolated atom density in the promolecule density, constructed by superimposing the isolated atom electron densities placed on the positions the atoms have in the molecule. A maximal conservation of the information of the isolated atoms in the atoms‐in‐molecules is thereby secured. Atomic charges, atomic dipole moments, and Fukui functions resulting from the Hirshfeld partitioning of the electron density are computed for a large series of molecules. In a representative set of organic and hypervalent molecules, they are compared with other commonly used population analysis methods. The expected bond polarities are recovered, but the charges are much smaller compared to other methods. Condensed Fukui functions for a large number of molecules, undergoing an electrophilic or a nucleophilic attack, are computed and compared with the HOMO and LUMO densities, integrated over the Hirshfeld atoms in molecules.

Systematic study of the parameters determining stockholder charges

Abstract The stockholder recipe to calculate atomic charges requires the definition of the so-called promolecule density. This density is defined in terms of the densities of the atoms of the molecule which are determined by their spectroscopic state and the level of theory used. In this study the basis set dependence, the effect of the atomic spectroscopic state and electron correlation upon stockholder charges are investigated.

brabo: a program for ab initio studies on large molecular systems

Abstract A combination of the multiplicative integral approximation with the direct SCF approach is implemented in the program brabo . This approach is an efficient combination for ab initio calculations on large molecular systems. For example, it was recently used to calculate the HF/4-21G gradient optimised structures of several hexapeptides (containing 56 atoms) and the structure and harmonic force fields of molecules in crystal clusters (involving 1092 basis functions).

Hirshfeld partitioning of the electron density: Atomic dipoles and their relation with functional group properties

Atomic dipole moments, derived within the Hirshfeld partitioning of the molecular electron density, have been studied for compounds of the type HX and ClX, for a series of functional groups X frequently encountered in organic molecules. In the case of the HX compounds, the component of the atomic dipole moment on H along the axis connecting H with the central atom in X is found to be linearly correlated with the electronegativity of X, the hardness of X playing no significant role. In the case of the ClX compounds, the situation is less clear. However, evidence seems to point to the conclusion that for these compounds, also the group hardness plays an important role.

Solids modeled by abinitio crystal field methods. X. Structure of α‐glycine, β‐glycine, and γ‐glycine using a 15‐molecule cluster

The structure of three polymorphic forms of glycine in the crystal phase, α‐glycine (P21/n), β‐glycine (P21), and γ‐glycine (P32), was completely optimized with standard gradient procedures using a point charge model and using a model constituted of a 15‐molecule cluster surrounded by point charges. The calculations were performed with a 6‐31G basis set and in the SCF step of the calculations the MIA approach was used. The results for the 15‐molecule cluster are in better agreement with the experimental results than the results obtained with the point charge model.

Atomic charges from modified Voronoi polyhedra

Abstract A method to calculate atomic charges from modified Voronoi polyhedra is presented. This method, a generalization of the method by Politzer and Harris (P. Politzer, R.R. Harris, J. Am. Chem. Soc., 92 (1970) 6451), yields charges of the same quality as the stockholder method (F.L. Hirshfeld, Theor. Chim. Acta (Berl), 44 (1977) 129), using discrete instead of diffuse boundaries.

Ab initio studies of crystal field effects : part VIII. structure of formamide oxime using a 15-molecule cluster

Abstract The structure of formamide oxime in the crystal phase (P2 1 2 1 2 1 ) was completely optimised with standard gradient procedures using a 15-molecule cluster surrounded by point charges. The calculations were performed using 4-21G and 6-31G basis sets and in the SCF step of the calculations the MIA approach was used. Results are in better agreement with experimental results than a previous study, in which only point charges were used to model the crystal.

LDA calculations of the Young's moduli of polyethylene and six polyfluoroethylenes

Abstract The Youngs moduli of the linear chain polymers, polyethylene and six polyfluoroethylenes, constrained to the planar conformation, are calculated by means of the local density approximation (LDA) using the Perdew–Zunger (PZ) correlation potential and a linear combination of Gaussian-type orbitals (LCGTO).

On the use of supermolecule model for calculation of Young's modulus of crystalline polymers

The use of the supermolecule model for the calculation of the Youngs modulus of crystalline polymers has been tested for a series of small and large n-alkanes. The molecules were divided into a head, body, and tail part, with the body being representative for a polyethylene chain. Calculations were performed with and without including periodicity in the body, and the results have been compared. Guidelines have been formulated to describe the applicability of the approach for other polymers.

Ab initio study of tetrahydro-imidazo[4,5,1-jk][1,4]-benzodiazepin-2(1H)-one and -thione (TIBO) derivatives R79882 and R82913

Abstract The gas phase structures of two TIBO compounds (R79882 and R82913), potent inhibitors of the reverse transcriptase of HIV 1, were studied with ab initio Hartree-Fock methods. For compound R82913 the geometries of a dimer and of the respective monomers were fully optimized and compared with experiment. For compound R79882 a complete geometry optimization of 15 different conformers was performed.