The source function, which enables one to equate the value of the electron density at any point within a molecule to a sum of atomic contributions, has been applied to a number of cases. The source function is a model-independent, quantitative measure of the relative importance of an atom's or group's contribution to the density at any point in a system, and it represents a potentially interesting tool to provide chemical information. It is shown that the source contribution from H to the electron density rho(b) at the bond critical point in HX diatomics decreases with increasing X's electronegativity, and that this decrease is a result of significant changes in the Laplacian distribution within the H-basin. It is also demonstrated that the source function from Li to rho(b) in LiX diatomics is a more sensitive index of atomic transferability than it is the lithium atomic energy or population. The observed changes are such as to ensure a constant percentage source contribution from Li to rho(b) throughout the LiX series, rather than a constant source as one would expect in the limit of perfect atomic transferability. Application of the source function to planar lithium clusters has revealed that the source function clearly discriminates between a nonnuclear electron density maximum and a maximum associated to a nucleus, on the basis of the relative weight of the source contributions from the basin associated to the maximum and from the remaining basins in the cluster. The source function has also allowed for a classification of hydrogen bonds in terms of characteristic source contributions to the density at the H-bond critical point from the H involved in the H-bond, the H-donor D, and the H-acceptor A. The source contribution from the H appears as the most distinctive marker of the H-bond strength, being highly negative for isolated H-bonds, slightly negative for polarized assisted H-bonds, close to zero for resonance-assisted H-bonds, and largely positive for charge-assisted H-bonds. The contributions from atoms other than H, D, and A strongly increase with decreasing H-bond strength, consistently with the parallel increased electrostatic character of the interaction. The correspondence between the classification provided by the Electron Localization Function topologic approach and by the source function has been highlighted. It is concluded that the source function represents a practical tool to disclose the local and nonlocal character of the electron density distributions and to quantify such a locality and nonlocality in terms of a physically sound and appealing chemical partitioning.

The experimental electron density of the high-performance thermoelectric material Zn4Sb3 has been determined by maximum entropy (MEM) analysis of short-wavelength synchrotron powder diffraction data. These data are found to be more accurate than conventional single-crystal data due to the reduction of common systematic errors, such as absorption, extinction and anomalous scattering. Analysis of the MEM electron density directly reveals interstitial Zn atoms and a partially occupied main Zn site. Two types of Sb atoms are observed: a free spherical ion (Sb3-) and Sb2(4-) dimers. Analysis of the MEM electron density also reveals possible Sb disorder along the c axis. The disorder, defects and vacancies are all features that contribute to the drastic reduction of the thermal conductivity of the material. Topological analysis of the thermally smeared MEM density has been carried out. Starting with the X-ray structure ab initio computational methods have been used to deconvolute structural information from the space-time data averaging inherent to the XRD experiment. The analysis reveals how interstitial Zn atoms and vacancies affect the electronic structure and transport properties of beta-Zn4Sb3. The structure consists of an ideal A12Sb10 framework in which point defects are distributed. We propose that the material is a 0.184:0.420:0.396 mixture of A12Sb10, A11BCSb10 and A10BCDSb10 cells, in which A, B, C and D are the four Zn sites in the X-ray structure. Given the similar density of states (DOS) of the A12Sb10, A11BCSb10 and A10BCDSb10 cells, one may electronically model the defective stoichiometry of the real system either by n-doping the 12-Zn atom cell or by p-doping the two 13-Zn atom cells. This leads to similar calculated Seebeck coefficients for the A12Sb10, A11BCSb10 and A10BCDSb10 cells (115.0, 123.0 and 110.3 microV K(-1) at T=670 K). The model system is therefore a p-doped semiconductor as found experimentally. The effect is dramatic if these cells are doped differently with respect to the experimental electron count. Thus, 0.33 extra electrons supplied to either kind of cell would increase the Seebeck coefficient to about 260 microV K(-1). Additional electrons would also lower sigma, so the resulting effect on the thermoelectric figure of merit of Zn4Sb3 challenges further experimental work.

The CH··O contacts in the 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione (DMACB) crystal have been characterized through a topological analysis of its experimental and theoretical densities, derived from a multipole refinement of X-ray diffraction data and from periodic Hartree−Fock calculations, respectively. The existence or the lack of an H··O bond critical pointthat is a point through the two nuclei where the gradient of the electron density vanishesallows us to distinguish between bonded and non bonded CH··O contacts, regardless of the value of their H··O separation. The 23 unique bonded contacts in DMACB are characterized by a large and nearly constant (∼140°) C−H−O angle, denoting the importance of the electrostatic energy contribution to such interactions. Instead, the nonbonded ones (four unique for H··O separations up to 3.0 Å) are more bent and may even be folded down to 90°, since their dominant van der Waals contribution to the interaction energy is independent of the C−H−O angle. The CH··O angular distribution observed for H··O separations greater than 2.7 Å is only apparently isotropic, since such isotropy clearly disappears when the bonded and nonbonded contacts are identified and their angular distributions separately analyzed. The Koch and Popelier criteria (J. Phys. Chem. 1995, 99, 9747) to establish H-bonds are, for the first time, applied in their entirety to a large set of CH··O contacts in a crystalline phase. The criteria are always satisfied by all of the bonded intermolecular CH··O contacts, with a single exception concerning one long bond and one of the six criteria only. The expressions proposed by Espinosa et al. (Chem. Phys. Lett. 1998, 285, 170), relating the potential energy densities at the critical point to the H-bond strengths, fail when applied to the weak CH··O interactions present in the DMACB crystal. The reasons for such a failure are outlined and new relationships are proposed. The importance of the promolecular charge distributions in defining topological properties of interest to the CH··O bonds is investigated. The criticism raised by Spackman (Chem. Phys. Lett. 1999, 301, 425) as to the lack of additional information provided by the experimental results to the description of such weak interactions is discussed. It is shown that the promolecular model yields significantly different electron density values at the critical point and in some instances even different topologies, compared to the corresponding multipole or theoretical densities. On the other hand, when the electron density topologies are the same, the values obtained from either electron density for the potential or kinetic energy density at the critical point, are very much alike.

We present a quantum Monte Carlo study of the solvation and spectroscopic properties of the Mg doped helium clusters MgHen with n = 2 − 50. Three high level (MP4, CCSD(T) and CCSDT) MgHe interaction potentials have been used to study the sensitivity of the dopant location on the shape of the pair interaction. Despite the similar MgHe well depth, the pair distribution functions obtained in the diffusion Monte Carlo simulations markedly differ for the three pair potentials, therefore indicating different solubility properties for Mg in Hen. Moreover, we found interesting size effects for the behavior of the Mg impurity.As a sensitive probe of the solvation properties, the Mg excitation spectra have been simulated for various cluster sizes and compared with the available experimental results. The interaction between the excited 1 P Mg atom and the He moiety has been approximated using the Diatomics-in-Molecules method and the two excited 1 Π and 1 Σ MgHe potentials. The shape of the simulated MgHe50 spectra show a substantial dependency on the location of the Mg impurity, and hence on the MgHe pair interaction employed.To unravel the dependency of the solvation behavior on the shape of the computed potentials, exact Density Functional Theory has been adapted to the case of doped Hen and various energy distributions have been computed. The results indicate the shape of the repulsive part of the MgHe potential as an important cause of the different behaviours.