A new method is introduced for the optimization of nonorthogonal virtual orbitals for use in general multiconfiguration spin-coupled wave functions. The use of a number of highly effective approximations greatly reduces the computational effort
involved, the most important being the use of a second-order perturbation expression for the energy and an approximate expression
for the elements of the Hessian. As a result, the overall scheme scales very favourably with respect to the numbers of active
electrons and of basis functions, making it suitable for the accurate study of large systems. Benchmark calculations are presented
for the dissociation of LiH(X1Σ+) and Li2(X1Σ+g) using a highly compact four-configuration wave function. Standard spin-coupled valence bond expansions in the same virtual
space are required to be significantly larger before equivalent results are obtained. The results are shown to compare very
favourably with full valence complete active space self-consistent field calculations using an identical basis, and binding
energies are within 4% of the values obtained from full configuration interaction calculations in the same basis set.
Received: 10 June 1997 / Accepted: 7 October 1997 相似文献
The chemical bonding in the refractory transition metal compounds TiC, TiN, and VN is investigated by experimental and theoretical techniques. High-precision X-ray diffraction is used to determine the electron densities in these three compounds experimentally. The X-ray structure factors and the respective valence electron densities are used twice, once to understand the chemical bonding and once to relate the experimental charge densities to those obtained from band structure calculations. These calculations, which in general are in very good agreement with experimental data, utilize the linearized augmented plane wave (LAPW) method. Theory and experiment lead to a detailed analysis of the chemical bonding in these compounds with 8, 9, and 10 valence electrons. By decomposition of the theoretical charge density into contributions from different states (energy regions), it was possible to show the strong covalent nonmetal p-metal d interaction, which is otherwise apparent only in TiC, but not in TiN or VN. In the latter two compounds the additional electrons occupy mainly metal d states with t2g symmetry, so that in the total valence charge densities the most important bonding feature is covered. In addition to covalent interactions all compounds have a metallic bonding contribution as well as a considerable charge transfer from the metal to the nonmetal site. This mixture in chemical bonding accounts for the unusual combination of properties such as ultrahardness, high melting points, and good conductivity. 相似文献
The distribution of valence and difference densities in crystalline CaMg(CO3)2 was calculated within the framework of the local density functional theory. It is shown that the terminal maxima of difference density located beyond the oxygen atom nuclei have different values due to the polarizing influence of the cations. 相似文献
Summary One-electron density matrices, which are representable in single-centers-orbital basis sets, have been investigated with respect to their reconstruction from densities. The maximum allowed dimension for reconstruction from a combination of position & momentum density dependent properties is only slightly bigger than the dimension in the case of position (or momentum) densities only. Since for a given one-particle basis of dimensionM, the number of one-matrix elements which can be determined is also of orderM only, while the total number of one-matrix elements is of orderM2, it is in general necessary to introduce severe constraints and restrictions. The accuracy demands on the data and algorithms increase exponentially for linearly increasing size of basis set. 相似文献
The electron localization function (ELF) is implemented in the first-principles, all-electron, full-potential local orbital method. This full-potential implementation increases the accuracy with which the ELF can be computed for crystalline materials. Some representative results obtained are presented and compared with the results of other methods. Although for crystal structures with directed bonding only minor differences are found, in simple elemental metals, there are differences in the valence region, which give rise to different ELF topologies. 相似文献
Quantum chemical calculations of the alkaline-earth oxides, imides and dihydrides of the alkaline-earth atoms (Ae=Be, Mg, Ca, Sr, Ba) and the calcium cluster Ca6H9[N(SiMe3)2]3(pmdta)3 (pmdta=N,N,N′,N′′,N′′-pentamethyldiethylenetriamine) have been carried out by using density functional theory. Analysis of the electronic structures by charge and energy partitioning methods suggests that the valence orbitals of the lighter atoms Be and Mg are the (n)s and (n)p orbitals. In contrast, the valence orbitals of the heavier atoms Ca, Sr and Ba comprise the (n)s and (n−1)d orbitals. The alkaline-earth metals Be and Mg build covalent bonds like typical main-group elements, whereas Ca, Sr and Ba covalently bind like transition metals. The results not only shed new light on the covalent bonds of the heavier alkaline-earth metals, but are also very important for understanding and designing experimental studies. 相似文献
Two valencies instead of one! Stoichiometric valence and structural valence are two distinct properties of atoms. The former, stoichV, is derived from the composition of a compound and has integer values; the latter, structV, depends on the structure of a compound and has non‐integer values. The scheme shows a representation of valence states of antimony and oxidation of SbIII to SbV, as a function of the eccentricity parameter Φ i.
In the current study, the coordination chemistry of nine-coordinate Ac(III) complexes with 35 monodentate and bidentate ligands was investigated using density functional theory (DFT) in terms of their geometries, charges, reaction energies, and bonding interactions. The energy decomposition analysis with naturals orbitals for chemical valence (EDA-NOCV) and the quantum theory of atoms in molecules (QTAIM) were employed as analysis methods. Trivalent Ac exhibits the highest affinities toward hard acids (such as charged oxophilic donors, fluoride), so its classification as a hard acid is justified. Natural population analysis quantified the involvement of 5f orbitals on Ac to be about 30% of total valence electron natural configuration indicating that Ac is a member of the actinide series. Pearson correlation coefficients were used to study the pairwise correlations among the bond lengths, ΔG reaction energies, charges on Ac and donor atoms, and data from EDA-NOCV and QTAIM. Strong correlations and anticorrelations were found between Voronoi charges on donor atoms with ΔG, EDA-NOCV interaction energies and QTAIM bond critical point densities. 相似文献
The calculation of quantum similarity measures from second-order density functions contracted to intracule and extracule
densities obtained at the Hartree-Fock level is presented and applied to a series of atoms, (He, Li, Be, and Ne), isoelectronic
molecules (C2H2, HCN, CNH, CO, and N2), and model hydrogen-transfer processes (H2/H+, H2/Hot, H2/H−). Second-order quantum similarity measures and indices are found to be suitable measures for quantitatively analyzing electron-pair
density reorganizations in atoms, molecules, and chemical processes. For the molecular series, a comparative analysis between
the topology of pairwise similarity functions as computed from one-electron, intracule, and extracule densities is carried
out and the assignment of each particular local similarity maximum to a molecular alignment discussed. In the comparative
study of the three hydrogen-transfer reactions considered, second-order quantum similarity indices are found to be more sensitive
than first-order indices for analyzing the electron-density reorganization between the reactant complex and the transition
state, thus providing additional insights for a better understanding of the mechanistic aspects of each process.
Received: 7 July 1997 / Accepted: 29 October 1997 相似文献
Densities of four aqueous H3BO3 solutions (0.062, 0.155, 0.315, and 0.529 mol-kg–1) have been measured in the liquid phase with a constant volume piezometer immersed in a precisely controlled liquid thermostat. Measurements were made at temperatures between 296 and 573 K and pressures from 0.82 to 48 MPa. The total uncertainties of the density, pressure, temperature, and molality measurements were estimated to be less than 0.06%, 0.05%, 10 mK, and 0.0005 mol-kg–1, respectively. The accuracy of the method was confirmed by PVT measurements on pure water for two isobars (30 and 39 MPa) at temperatures from 313 to 573 K. The experimental and calculated (IAPWS formulation) densities for pure water show excellent agreement which is within their experimental uncertainties (average absolute deviation, AAD=0.012%;). Apparent and partial molar volumes were derived using the measured densities for solutions and pure water, and these results were extrapolated to zero concentration to yield the partial molar volumes of the electrolyte (H3BO3) at infinite dilution. The temperature, pressure, and concentration dependencies of the apparent and partial molar volumes were studied. Small pressure and concentration effects on the apparent molar volumes were found at temperatures up to 500 K. The parameters of a polynomial type of equation of state for the specific volume Vsol(P, T, m) as a function of pressure, temperature, and molality were obtained with a least-squares method using the experimental data. The root-mean-square deviation between measured and calculated values from this polynomial equation of state is ±0.2 kg-m–3 for density. Measured values of the solution densities and the apparent and partial molar volumes are compared with data reported in the literature. 相似文献
We have created a set of crystalline model structures exhibiting straight lines of Al3+ connected to chalcogenides (O2−, S2−, and Se2−) connected to metal cations of varying valence (Sr2+, Y3+, Zr4+, Nb5+, and Mo6+). They were relaxed with density functional theory computations and analysed by Bader partitioning. As Al3+ ions are supposed to strongly interact with their atomic environment, we studied the electron density topology induced by higher-valent cations in the extended chemical neighbourhood of Al. In fact, we found a general decrease of ionic charges and an increasing displacement of the chalcogenides towards higher-valent ions for the heavier chalcogens. Therefore, we comprehensively screened S- and Se-containing compounds for candidates theoretically exhibiting low migration barriers for Al3+ ions by using Voronoi–Dirichlet partitioning and bond valence site energy calculations. The basis for this search is the Inorganic Crystal Structure Database. Indeed, we could extract six promising candidates with low Al3+ migration barriers. which are even lower than the barriers for any other element inside of these materials. This will encourage efforts towards preparing suitable Al3+ conductors. 相似文献
Intermediate valence behavior is frequently observed in materials containing Ce, Yb, Eu, Sm, or Tm. In the current work, we report synthesis and characterization of Yb2Cu9Al8 (Th2Zn17 structure type). Its intermediate valence behavior can be described by an excitation energy Eex/kB = 319 K and a spin fluctuation temperature Tsf = 60 K. The valence state of Yb is estimated to be close to 2.04 for the low-temperature region. The valence gradually evolves to the value of 2.80 at T = 400 K. The specific heat coefficient of γexp = 59 mJ · molYb–1 · K–2 indicates a moderate effective mass enhancement, together with finite density of states at the Fermi level. The latter is also confirmed by the band structure calculations. 相似文献