Pseudoatom expansions of the first-row diatomic hydride electron densities

Abstract

Representations of high-quality molecular electron densities are studied. An evaluation of restricted radial functions is made using a least-squares figure of merit, the molecular dipole and quadrupole moments, the electric fields at the nuclei, the electric-field gradients at the nuclei, an approximate energy and differencedensity maps. For the heavy atom, a satisfactory representation has a fixed core function with a variable population and requires optimized dipolar core polarization functions, and an additional monopole term. The heavy-atom valence regions, and the H require expansions to at least the quadrupole level, with one Slater-type radial function per multipole and all exponents optimized. Additional valence radial functions and higher multipoles are required to give completely satisfactory difference-density maps but do not consistently improve the physical properties.

On the transferability of QTAIMC descriptors derived from X-ray diffraction data and DFT calculations: substituted hydropyrimidine derivatives

“…Tsirelson et al (2006) also made an accurate joint low-temperature X-ray diffraction and nonempirical quantum chemical study of bonding in (3), and obtained extensive bond and atomic quantum-topological descriptors. A comparison of the QTAIMC results obtained for (3) from accurate X-ray diffraction measurements, threedimensional periodic calculations (CRYSTAL98; Saunders et al, 1998) and single-molecule DFT B3LYP/6-311G(d,p) calculations confirmed earlier findings (Chandler & Spackman, 1982;Parini et al, 1985;Iversen et al, 1997;de Vries et al, 2000;Volkov et al, 2000;Volkov & Coppens, 2001;Flaig et al, 2002;Bytheway et al, 2002;Tsirelson et al, 2006) that the main differences between the experimentally and theoretically obtained properties are observed for the longitudinal ('along the bond path') ED curvature, 3 . This may be attributed to a deficiency of the ED description with current multipole models.…”

Molecular and crystal properties of ethyl 4,6-dimethyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate from experimental and theoretical electron densities

“…A significant discrepancy in the model and wavefunctionbased Laplacians of the electron density for shared interactions has already been noted in the literature (Bianchi et al, 1996;Spackman et al, 1999;Flaig et al, 2002;Zhurova et al, 2004;Henn et al, 2004;Hibbs et al, 2005). Insufficient flexibility of existing multipole models to span the charge distribution both close to the nuclei and at the middle-bond area (especially for polar covalent bonds) is considered to be the main reason for this discrepancy (Chandler & Spackman, 1982;Parini et al, 1985;Iversen et al, 1997;de Vries et al, 2000;Volkov et al, 2000;Volkov & Coppens, 2001), the maximum discrepancy being found for the ED curvature along the bond line, 3 ( Table 4 Atomic volumes (), charges (Q) and electronic energies (H) integrated within the atomic basins.…”

Contributions to the application of the transferability principle and the multipolar modeling of H atoms: electron-density study ofL-histidinium dihydrogen orthophosphate orthophosphoric acid. I

“…In order to explore the best procedure to obtain the electron-density distribution around the H atoms, two X-(X+N) refinements were performed. In the first one (XL-Na refinement), the H-atom multipolar expansion was extended up to the quadrupolar term, as recommended by Chandler & Spackman (1982). In the other refinement (called XL-Nb), the multipolar expansion was truncated at the dipolar level.…”