The molecular electron density distribution meeting place of X-ray diffraction and quantum chemistry intermediate - between theory and experiment

Quantum chemistry and the concepts used daily in chemistry are increasingly growing apart. Among the concepts that are able to bridge the gap between theory and experimental practice, electron density distribution has an important place. The study of this distribution has led to new developments in theory, including Hellmann-Feynman theory and the density functional theory. The possibilities and limitations of these methods are discussed. Various ways of analysing the electron density distribution are presented and discussed. X-ray diffraction enables us to ¿observe¿ the electron density distribution and electrostatic properties. Experimental results are compared with the results of quantum chemical calculations. It is shown that even intermolecular interaction is observable with this method. Problems in determining ionic charges are seen to be inherent in the method

The shielding of external electric fields in atoms revisited

An atom, placed in an external homogeneous field, will show a complex charge distribution. The pattern of the polarization density distribution, obtained by subtracting the original electron from the one of the polarized atom, can easily be explained by considering the various orbitals. Poisson's equation relates the induced field to polarization density distribution

The crystal structure of urea nitrate

The structure of urea nitrate has been solved, by the use of three-dimensional X-ray data. Data were collected using Cu Ke and Mo K0~ radiations. The structure consists of layers with urea and nitrate groups held together by hydrogen bonds. The positions of all hydrogen atoms were found. The final R values for Cu and Mo measurements are 4.8% and 6.2% respectively. The agreement between the two sets of data is good

The electron density distribution in CN−, LiCN and LiNC. The use of minimal and extended basis set SCF calculations

Electron density maps are reported for the CN−ion and the LiCN and LiNC molecules, calculated from molecular wave-functions near the Hartree-Fock limit. The electron density distribution derived from CNDO/ 2 wavefunctions does not resemble the ab initio results. The ultimate ability of a minimal basis set to represent the electron density near the Hartree-Fock limit, has been tested. The requirement of N-representability of the trial electron density has been satisfied. It is found that the molecular valence density cannot be reproduced to a satisfactory extent by a minimal set of Slater orbitals, even when the exponents of the basis orbitals are optimized

Accuracy of various approximations to exchange and correlation for the electron density distribution in atoms and small molecules

The general usefulness of various local and non-local approximations to the exchange-correlation potential in density functional theory is studied by comparing resulting electron density distributions to essentially exact results for light atoms. The correlation contribution to the electron density in CO and H2O is compared with CI results. It is concluded that density functional theory provides a viable alternative to HF and CI approaches for the calculation of deformation densities, although the response of the electron density to the correlation potential is only moderately accurate

A modified plane wave model for calculating UV photo-ionization cross-sections

Photoionization cross-sections are calculated for a number of molecules, using a plane wave method. Agreement with experimental data is considerably improved with respect to common plane wave results if the energy of the photoelectron is assumed to equal the incident photon energy

Uncoupled Hartree-Fock calculations of the polarizability and hyperpolarizabilities of nitrophenols

The polarizability and hyperpolarizabilities of nitrophenols as model compounds for studying nonlinear optics have been investigated at the Hartree-Fock level of approximation by means of the Dalgarno Uncoupled Hartree-Fock (DUHF) or Sum Over Orbitals (SOO) method. The additive character and the charge transfer effects in α,β,γ and have been analyzed in terms of the δ and π molecular orbital contributions, the contribution of the individual π molecular orbitals, and the contribution of the highest occupied and the lowest unoccupied\ud molecular orbitals. Within the SOO approach, the reliability of the Two-Level Model has been tested and the influence of the rotation of the nitro group and of the presence of the intramolecular hydrogen bonding in ortho-nitrophenol have been studied. The results show that the present method is a reliable and efficient tool for the prediction of trends in the molecular polarizability and hyperpolarizabilities of large molecule

From wave function to crystal morphology: application to urea and alpha-glycine

In this paper the relation between the molecular electron density distribution and the crystal growth morphology is investigated. Accurate charge densities derived from ab initio quantum chemical calculations were partitioned into multipole moments, to calculate the electrostatic contribution to the intermolecular interaction energy. For urea and alpha-glycine the F-faces or connected nets were determined according to the Hartman-Perdok PBC theory. From attachment energy and critical Ising temperature calculations, theoretical growth forms were constructed using different atom-atom potential models. These were compared to the Donnay-Harker model, equilibrium form and experimental growth forms. In the case of alpha-glycine, the theoretical growth forms are in good agreement with crystals grown from aqueous solution. Crystals obtained by sublimation seem to show some faces which are not F-faces sensu stricto

The crystal structure of thiourea nitrate

The structure of thiourea nitrate has been determined by three-dimensional X-ray methods. Both Cu Ke and Mo K~ data were obtained with a single-crystal diffractometer and the final R values are 4.9% and 5.5% respectively. The space group is P211m and there are two molecules in the unit cell. All atoms, including hydrogen, lie on mirror planes. The whole structure is built up of layers of atoms, the atoms within each layer being linked by a network of hydrogen bonds

Molecular charge distribution of CO

The difference electron density of CO is studied by comparison of several calculations. It is shown that the Hartree-Fock-Slater and Hartree-Fock methods yield equally good charge-distributions and that the use of minimal basis sets should be avoided