Ab initio and XPS studies of pyrite (1 0 0) surface states

Ab initio quantum chemical modelling (GGA, CASTEP and B3LYP, CRYSTAL03) is used to predict differences in electronic structure between the (1 0 0) surface and bulk of pyrite. Experimental X-ray photoelectron spectroscopic (XPS) data for the S 2p core lines show the presence of two types of S surface states: surface S²⁻ monomers at a S 2p_3/2 binding energy (BE) of 161.2 eV, and (S–S)²⁻ surface dimer states at a S 2p_3/2 BE of 162.0 eV, compared to the S 2p_3/2 BE of bulk pyrite at 162.7 eV. The Fe 2p surface XPS displays several multiplets (implying high spin configuration) at higher BE than the bulk Fe 2p signal, which can be ascribed to surface state contributions. The quantum chemical simulation predicts an S 2p core level shift of 0.69 eV between the S bulk and S surface dimers, in good agreement with the 0.6 eV found in XPS measurements. A Mulliken population analysis confirms the conjectured charge distribution on the surface, which leads to the two different S surface states, as well as the surface high spin configuration responsible for the high BE Fe multiplets. Evidence for surface Fe²⁺ and Fe³⁺ surface states can be seen in the Fe projected valence band density of states, confirming the interpretation of the photoemission spectra.     

Ab initio MO-SCF calculation on a model of anomalous water

Ab initio calculations are carried out on the planar hexamer model of anomalous water using a variety of basis sets. The results are compared to results of similar calculations on free water and water in the ice I-like puckered ring configurations.

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Zusammenfassung Es werden ab initio-Rechnungen für das planare Hexamer-Modell von anomalem Wasser unter Verwendung mehrerer BasissÄtze durchgeführt. Die Ergebnisse werden mit Resultaten Ähnlicher Rechnungen über freies Wasser und Wasser in der I-Eis Ähnlichen Konfiguration verzerrter Ringe verglichen.

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Résumé Calculs ab initio sur le modèle hexamérique plan de l'eau anormale en utilisant différentes bases. Les résultats sont comparés à ceux de calculs similaires sur l'eau libre et sur l'eau dans une configuration cyclique compacte analogue à celle de la glace I.

     

Ab initio density functional study of MgO (001) surface with topological defects

Magnesium oxide (MgO) is a useful material because of its catalytical properties. Moreover, surface defects have been found to play a crucial role in determining the activity and selectivity of catalytic surfaces. We present the results of a theoretical investigation of the electronic structure of the MgO (001) surface with three different topological defects. Our calculations are based on density functional theory (DFT) and the pseudopotential method. Slab geometry and periodic boundary conditions have been included with occupied orbitals expanded in plane waves. We compare the obtained results of the (001) MgO clean surface with those containing topological defects. The density of states DOS is analyzed in each case, as well as the chemical nature of the atoms belonging to the defects. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Ab initio molecular orbital calculation of fe-porphine with a double zeta basis set

An ab initio LCAO SCF MO calculation was performed on planar Fe-porphine with a double zeta basis set consisting of 300 CGTO 's. SCF wave functions of several states of Fe-porphine and its cation were obtained. The net charge of Fe is in the range of 1.39 to 1.53. The highest occupied orbital is ascertained to be a pure porphine π-MO , 1a_1u. The calculated ionization potentials of the two highest occupied orbitals, 1a_1u and 5a_2u are 5.98 and 6.43 eV, respectively. They are in good agreement with experiments. The role of the porphine macrocycle on the oxidation of Fe is discussed in terms of gross atomic populations and with contour maps of the density difference.     

Ab initio calculation of ESCA satellite intensities for a copper–chlorine complex

Ab initio SCF calculations carried out on the planar cluster CuCl₄^2? suggest that the strong satellites at the high-binding energy side of inner-shell ESCA lines of Cu are due to charge relaxations in the σ-bonding orbitals which mix with Cu 3d. The satellite state with the largest intensity may be described as a one-electron excitation Cu 3d → Cl 3p.     

Ab initio calculation of magnetic shielding and susceptibility. Finite perturbation calculation of susceptibility with GIAO's

The magnetic susceptibility of hydrogen fluoride is calculated with four different basis sets of gauge invariant atomic orbitals (GIAO's). The Roothaan equations are solved for various values of the magnetic field strength and the susceptibility is deduced by a numerical differentiation of the energy.     

Ab initio EPR study of S and Se defects in alkali halides

Calculations using density functional theory are performed to study the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) properties of S and Se impurities in alkali halide lattices. Cluster in vacuo models are used to describe the defect and the lattice surroundings. The trivacancy defect model proposed in the literature is able to reproduce both the experimental principal values and directions of the g tensor for S and Se defects doped in alkali halides. The alternative monovacancy model gives rise to important discrepancies with experiment and can be discarded. For the KCl lattice, the hyperfine tensors of the S and Se molecular ions also agree well with the available experimental data, giving further evidence to the trivacancy model. In addition, for NaCl:S and KCl:S computational results for the ²³Na and ³⁵Cl superhyperfine and quadrupole tensors are compared with experimental ENDOR parameters. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

2-苯基苯并咪唑衍生物非线性光学性质的从头算研究

采用HF／6-31G^*方法优化分子构型，在此基础上用CPHF方法系统地研究了多种基团取代的2-苯基苯并咪唑衍生物的二阶非线性光学系数βvec，并对βvdc的影响因素进行了探讨，为进一步设计综合性能优良的有机非线性光学材料提供理论指导．     

Ab initio calculation of molar volumes: Comparison with experiment and use in solvation models

A simple and efficient procedure of calculating molecular volume (V_M) based on the Monte Carlo method is presented. The volume of a molecule is defined by the volume occupied by the 0.001-au electron density envelope. We have employed this method to compute the molecular volumes (V_M) of a large selection of organic molecules and compare them with the corresponding molar volumes (V_m) measured in the liquid state. A strong correlation is found to exist between the V_M and V_m values (V_M/V_m ≈ 0.75). Using this linear relationship, the calculated molecular volume may provide an estimate of the cavity-volume parameter in solvent-effect calculations. As a chemical application of molecular volume, we have investigated the conformational equilibrium of 1,2-difluoroethane in the liquid state using the self-consistent reaction field theory. © 1995 by John Wiley & Sons, Inc.     

Ab initio calculation of spin–orbit interaction in polyatomic molecules using Gaussian-type wavefunctions

A set of programs has been developed to calculate molecular spin–orbit interaction with Gaussian-type wavefunctions in connection with the popular GAUSSIAN 76 program. The spin–orbit contributions to the fine structure of O₂ (X³∑_g^?), NH (X³∑^?), and CH₂ (X³B₁) are evaluated with the standard STO -3G and 6-31G basis sets; for NH the influence of bond functions added to the latter basis set is also investigated. The results are compared to values previously obtained with other types of basis sets.     

Ab initio study of K adsorption on a Ag(100) surface

Ab initio Hartree–Fock calculations are reported for the chemisorption of K on Ag using three different types of clusters to model the system. Geometry optimization is done in 4 degrees of freedom. It is found that since there is an absence of complete charge transfer between the adsorbate and substrate, the interaction can be interpreted as being predominately covalent in nature.     

Ab initio study on the adsorption and dissociation process of methanol on a silver surface

The geometries of methanol adsorbed on an oxygen-free silver surface, a promoted silver surface and an oxygen preadsorbed silver surface were optimized at the MP2 level and the energies were calculated at the MP4 level. Our calculations showed that weak physisorption of methanol occurs on the clean silver surface, but stable molecular chemisorption occurs in the other two cases. The adsorption and dissociation process of methanol was postulated to occur via two pathways, i.e. the Eley-Rideal mode and the Langmuir-Hinshelwood mode. The calculations also showed that the presence of atomic oxygen at a silver surface is essential for the cleavage of the OH bond in the methanol. The dissociation of methanol in the Langmuir-Hinshelwood mode has a small energy barrier but has no energy barrier in the Eley-Rideal mode.     

Ab initio calculation of torsion and inversion barriers of the amino group in aminopyrimidines

Calculations of the barriers to internal rotation and inversion of the amino group in substituted pyrimidines have been performed. Torsion and inversion barriers were determined by several ab initio methods: HF, HF/MP2, MP4, CISD, QCISD, QCISD(T), CCSD, and CCSD(T). DFT method also employed. Dependencies of the calculated barrier heights on the basis set and the electron correlation level and on the substitution position of the nitrogen atom in the ring were studied. We have determined that for certain molecules relatively low level calculations may eventually provide adequate results, but in general, higher level calculations are necessary.     

氢原子在金属锂表面台阶附近吸附和表面扩散行为的ab initio研究

取Li7H和Li9H两个原子簇模拟氢原子与含台阶的金属锂表面的相互作用, 以小基组用ab initip方法计算了体系的吸附和表面扩散势能面(或势能曲线)。结果表明: (1)对Li7H体系, 台阶面附近沿垂直边棱方向存在三种不同的桥位吸附位, 最稳定的吸附位在上台面接近台阶边棱处, 台阶面显著地改变了表面扩散活化能, 台阶边棱处有一个较高的势垒。于是, 迁移原子将会在台阶边棱处受到反射, 并可被捕获于台阶面上及其附近。由势能面确定了最低能量表面扩散途径。(2)对Li9H体系, 在Li7H原子簇基础上增加次表面层两个锂原子后, 表面扩散活化能略有减小, 氢原子在上台面的桥位吸附更趋稳定, 各吸附位相对稳定性及势垒内何位置几无改变, 这些结果显示了台阶面对氢原子的化学吸附和表面扩散发生扰动, 台阶边棱对表面扩散起着重要作用。     

Ab initio potential energy surface and vibration‐rotation energy levels of sulfur dioxide

An accurate potential energy surface of sulfur dioxide, SO₂, in its ground electronic state has been determined from ab initio calculations using the coupled‐cluster approach in conjunction with the correlation‐consistent basis sets up to septuple‐zeta quality. The results obtained with the conventional and explicitly correlated coupled‐cluster methods are compared. The role of the core–electron correlation, higher‐order valence–electron correlation, scalar relativistic, and adiabatic effects in determining the structure and dynamics of the SO₂ molecule is discussed. The vibration‐rotation energy levels of the ³²SO₂ and ³⁴SO₂ isotopologues were predicted using a variational approach. It was shown that the inclusion of the aforementioned effects was mandatory to attain the “spectroscopic” accuracy. © 2017 Wiley Periodicals, Inc.     

Ab initio potential energy surface and vibration‐rotation energy levels of beryllium monohydroxide

The accurate potential energy surface of beryllium monohydroxide, BeOH, in its ground electronic state has been determined from ab initio calculations using the coupled‐cluster approach in conjunction with the correlation‐consistent core‐valence basis sets up to septuple‐zeta quality. The higher‐order electron correlation, scalar relativistic, and adiabatic effects were taken into account. The BeOH molecule was confirmed to be bent at equilibrium, with the BeOH angle of 141.2° and the barrier to linearity of 129 cm⁻¹. The vibration‐rotation energy levels of the BeOH and BeOD isotopologues were predicted using a variational approach and compared with recent experimental data. The results can be useful in a further analysis of high‐resolution vibration‐rotation spectra of these interesting species. © 2016 Wiley Periodicals, Inc.     

Ab initio molecular orbital calculation on the rotational isomerism of 1,2-dichloroethane and 1-chloropropane

Ab initio MO calculations on the total energies of the rotational isomers of 1,2-dichloroethane and 1-chloropropane were performed to estimate the energy differences and the potential functions. The torsional energy levels were calculated by the harmonic rotator model. The calculation including the configuration interaction with single and double excitations on 1,2-dichloroethane improved the agreement with the experimental energy difference between the trans and gauche forms. Vibrational frequencies of 1,2-dichloroethane were also calculated by the ab initio MO method. The results are in good correlation with experimentally established values by Shimanouchi.     

四唑及其衍生物的理论研究 2: 氯代四唑负离子的从头算

用从头计算法, 取6-21G基组, 在MP2水平上, 计算研究了1H-和2H-四唑一氯取代物三种负离子的全优化几何构型和电子结构, 比较讨论了它们的芳香性和稳定性。发现三者均取平面构型, 其芳香性和稳定性次序为: 5-氯四唑负离子>2-氯四唑负离子>1-氯四唑负离子。预示了形成金属配合物时5-氯四唑作为配体的重要性和配位方式。     

Ab initio calculation for inner reorganization energy of gas-phase electron transfer in organic molecule–ion systems

A series of electron transfer (ET) reactions between some organic molecules have been investigated through ab initio calculations. Biphenyl (Bp) and 9,9^′-dimethylfluorene anion radicals are chosen as the donor, whereas several organic molecules with different redox abilities are chosen as the acceptor. The inner reorganization energy and the endothermicity of the ET reactions in those molecule–ion systems have been estimated through the HFSCF and complete active space multiconfiguration SCF calculations. Double-well potentials for the gas-phase ET reactions have been constructed using the linear reaction coordinate, and the results show that the quinone-containing ET reactions are in Marcus' inverted region. It has been found that the inner reorganization energies are different for various donor-acceptor couples, unlike the experimentally fitted ones. The contribution from the inter-ring torsional motion in Bp to the inner reorganization energy has been evaluated from the energy difference of the biphenyl-acceptor and the dimethylfluorine-acceptor systems. Comparisons with the experimentally observed results have been made.