Implementation of molecular symmetry in valence bond calculation

A novel strategy for the construction of many-electron symmetry-adapted wave function is proposed for ab initio valence bond (VB) calculations and is implemented for valence bond self-consistent filed (VBSCF) and breathing orbital valence bond (BOVB) methods with various orbital optimization algorithms. Symmetry-adapted VB functions are constructed by the projection operator of symmetry group. The many-electron symmetry-adapted wave function is expressed in terms of symmetry-adapted VB functions, and thus the VB calculations can be performed with the molecular symmetry restriction. Test results show that molecular symmetry reduces the computational cost of both the iteration numbers and CPU time. Furthermore, excited states with specific symmetry can be conveniently obtained in VB calculations by using symmetry-adapted VB functions.     

Analysis of the convergence of the general coupling operator method for one-configuration-type wave functions

We comment on the convergence of the general coupling operator for all types of one-configuration or multiconfigurational wave functions that still preserve the one-configuration structure for the energy expression. The choice on the best arbitrary real and antisymmetric parameters inherent in the coupling operator methodology is discussed, giving a theoretical reason. Another type of coupling operator is defined, presented, and analyzed. Finally, we give some numerical examples related to the low-lying electronic states of a cluster model for K₂NiF₄ solid. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 368–376, 1998     

A multireference configuration interaction method based on the separated electron pair wave functions

A multireference configurational interaction method based on the separated electron pair (SEP) wave functions, SEP‐CI approach, has been developed as an approximation to the traditional CASSCF method. It differs from the CASSCF method in that active orbitals are obtained from the SEP wave function without further optimization in the subsequent CI calculations, and the active space is automatically constructed according to the occupation coefficients of SEP natural orbitals. These features make the present SEP‐CI method computationally much less demanding than the CASSCF method. The applicability of the SEP‐CI method is illustrated with sample calculations on the insertion reaction of BeH₂ and dissociation energies of LiH, BH, FH, H₂O, and N₂. © 2005 Wiley Periodicals, Inc. J Comput Chem 27: 39–47, 2006     

Exponentially generated wave functions and excited states of benzene

Usefulness of the exponentially generated wave function approach is shown. We first give an overview of the SAC (symmetry adapted cluster) and SAC-CI study on the valence and Rydberg excitations and ionizations of benzene including both and spaces. The importance of the reorganization effect is found for the T₃(³B_2u), S₂(¹B_1u), and S₃(¹E_1u) states, so-called V states. A first systematic calculation is reported for the Rydberg excited states. Next, the idea of the exponentially generated wave function (EGWF) theory is explained. New exponential-type operators and new wave functions associated with them are defined. The mixed or multi use of these exponential operators is shown to be effective both physically and practically. We call the resultant wave functions MEG (multi-exponentially generated) wave functions. We then explain the algorithm of calculations and show some results on the potential energy curves of the ground, excited, and quasi-degenerate states of some diatomics and triatomics.     

Analytic expression of the second derivatives of electronic energy for full configuration interaction wave functions

Summary A new analytic second derivative expression of the electronic energy is derived for full configuration interaction (CI) wave functions. This formula is shown to be free from the derivative terms of both CI and MO coefficients. The second-order relationships between CI and MO coefficients for full CI wave functions are also presented.     

Trial wave functions induced by the minimum mean deviation from statistical equilibrium

The ground state of an atom is studied by introducing a trial function whose unknown coefficients, taken as variational parameters, are determined by minimizing the mean energy of the respective quantum system. There is no clear methodology about how to choose such trial functions. Using techniques from information theory and statistical inference, this study deals with the construction of trial wave functions by minimizing the mean deviation from statistical equilibrium. The formalism is applied to the ground state of the helium and lithium atoms with surprisingly good numerical results even when only a few variational parameters are used. Details about the effective computation are also given. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 175–190, 1998     

Charge decomposition analysis of the electron localizability indicator: a bridge between the orbital and direct space representation of the chemical bond

The novel functional electron localizability indicator is a useful tool for investigating chemical bonding in molecules and solids. In contrast to the traditional electron localization function (ELF), the electron localizability indicator is shown to be exactly decomposable into partial orbital contributions even though it displays at the single-determinantal level of theory the same topology as the ELF. This approach is generally valid for molecules and crystals at either the single-determinantal or the explicitly correlated level of theory. The advantages of the new approach are illustrated for the argon atom, homonuclear dimers N2 and F2, unsaturated hydrocarbons C2H4 and C6H6, and the transition-metal-containing molecules Sc(2)2+ and TiF4.     

油页岩干酪根化学键浓度与能量密度研究

以抚顺、茂名油页岩干酪根13C NMR、XPS与元素分析数据为基础,构建了油页岩干酪根分子结构模型,同时以化学键为标准对抚顺、茂名干酪根结构模型进行了修改,构建的干酪根结构模型与实验化学键浓度匹配良好,从化学键角度验证了模型的准确性与合理性。以自建及文献中九个不同变质程度的油页岩干酪根结构模型为基础,研究了油页岩干酪根变质程度与各类化学键浓度及能量密度关系。结果表明,随油页岩干酪根变质程度的提高,芳香碳分别与芳香碳、脂肪碳、氢原子等原子形成的化学键浓度升高,脂肪碳与脂肪碳、氢原子等原子形成的化学键浓度下降,其中,芳香碳之间、脂肪碳与氢原子之间的化学键浓度变化最明显。组成油页岩干酪根势能的价电子能密度及非键能密度随干酪根变质程度的提高总体上呈现上升趋势,成为组成油页岩干酪根稳定的化学能。     

维生素B2的极谱催化波研究及应用

维生素B2是人和动物维持机体正常结构与功能的必需营养物质,具有广泛的生理功能。目前,测定维生素B2的方法有高效液相色谱法、荧光分析法、化学发光分析法、紫外分光光度法及电化学分析法等,而其中电化学分析法大都需要采用化学修饰电极,对电极需特殊处理,比较麻烦和费时。本文发现在KH2PO4-Na2HPO4缓冲介质中,维生素B2在-0．43V（vs．SCE）电位附近产生的还原波可被H2O2催化,产生一新的极谱催化波。通过伏安分析并结合光谱分析,本文研究并推导了该极谱催化波的产生机理,并建立了测定维生素B2的新方法。该方法简单,操作简便快速,无需对电极进行特殊处理,而灵敏度与其他电化学测定方法相当。     

Construction of database of effective energy‐loss functions

Effective energy‐loss functions for Al, Cu, Ag and Au were derived from the reflection electron energy‐loss spectroscopy (REELS) spectra for 1 keV electrons using extended Landau theory. Features of the obtained effective energy‐loss functions are close to those of optical surface energy‐loss functions, revealing the significant contribution of the low energy loss below a few tens of electron‐volts in the REELS spectrum for Cu, Ag and Au. The REELS spectra were reproduced using the newly derived effective energy‐loss functions, leading to the confirmation that this type of database of the effective energy‐loss function is very useful not only for more comprehensive understanding of the measured spectrum of surface electron spectroscopies but also for practical background subtraction in surface electron spectroscopy. Copyright © 2003 John Wiley & Sons, Ltd.     

Detailed comparison of the pnicogen bond with chalcogen,halogen, and hydrogen bonds

The characteristics of the pnicogen bond are explored using a variety of quantum chemical techniques. In particular, this interaction is compared with its halogen and chalcogen bond cousins, as well as with the more common H‐bond. In general, these bonds are all of comparable strength. More specifically, they are strengthened by the presence of an electronegative substituent on the electron‐acceptor atom, and each gains strength as one moves down the appropriate column of the periodic table, for example, from N to P to As. These noncovalent bonds owe their stability to a mixture in nearly equal parts of electrostatic attraction and charge transfer, along with a smaller dispersion component. The charge transfer arises from the overlap between the lone pair of the electron donor and a σ* antibond of the acceptor. The angular characteristics of the equilibrium geometry result primarily from a compromise between electrostatic and induction forces. Angular distortions of the H‐bond are typically less energetically demanding than comparable bends of the other noncovalent bonds. © 2012 Wiley Periodicals, Inc.     

Observation of Transition‐Metal–Boron Triple Bonds in IrB2O− and ReB2O−

Multiple bonds between boron and transition metals are known in many borylene (:BR) complexes via metal d_π→BR back‐donation, despite the electron deficiency of boron. An electron‐precise metal–boron triple bond was first observed in BiB₂O^? [Bi≡B?B≡O]^? in which both boron atoms can be viewed as sp‐hybridized and the [B?BO]^? fragment is isoelectronic to a carbyne (CR). To search for the first electron‐precise transition‐metal‐boron triple‐bond species, we have produced IrB₂O^? and ReB₂O^? and investigated them by photoelectron spectroscopy and quantum‐chemical calculations. The results allow to elucidate the structures and bonding in the two clusters. We find IrB₂O^? has a closed‐shell bent structure (C_s, ¹A′) with BO^? coordinated to an Ir≡B unit, (^?OB)Ir≡B, whereas ReB₂O^? is linear (C_∞v, ³Σ^?) with an electron‐precise Re≡B triple bond, [Re≡B?B≡O]^?. The results suggest the intriguing possibility of synthesizing compounds with electron‐precise M≡B triple bonds analogous to classical carbyne systems.     

Comparison of energy‐loss functions from REELS spectra with surface and bulk energy loss functions for Ag

Effective energy‐loss functions were derived from the reflection electron energy‐loss spectroscopy (REELS) spectra of Ag by an extended Landau approach. The effective energy‐loss functions obtained are close to the surface energy‐loss function in the low‐energy‐loss region, but tend to be closer to the bulk energy‐loss function in the higher energy‐loss region for higher primary energy. The REELS spectra incorporating the effective energy‐loss function are also reproduced in a Monte‐Carlo simulation model and confirm that the simulation reproduces the experimental REELS spectra with considerable success. Copyright © 2003 John Wiley & Sons, Ltd.     

Transition probability of lithium atom and lithiumlike ions with weakest bound electron wave functions and coupled equations

One of us has presented a new theory called the Weakest Bound Electron Potential Model theory (WBEPM theory) to determine the exact potential field in atoms. A concise analytic form of the potential of the weakest‐bound electron is proposed. In the new potential and the wave function derived afterward, the integral atomic number Z and quantum numbers n and l are replaced by nonintegral Z*, n*, and l*, respectively. In this article, we employed the WBEPM theory to calculate the transition probability for the lithium atom and lithiumlike ions and obtained very good results compared with the accepted values and other results. © 1999 John Wiley & Sons, Inc. Int J Quant Chem 76: 51–61, 2000     

Effective way of modeling chemical catalysis: Empirical valence bond picture of role of solvent and catalyst in alkylation reactions

A general methodology for the study of chemical catalysis is presented and demonstrated in a study of Friedel–Crafts‐type alkylation reactions that are constrained to collinear configurations. Ab initio potential energy surfaces in solution and relevant experimental results are used to calibrate general empirical valence bond (EVB) potential surfaces for studies of such reactions. The EVB surfaces allow one to interpolate the ab initio results to studies of the effect of different solvents, substituents, and catalysts on the alkylation reactions. This implicit approach introduces such effects by shifting the diagonal energies of the corresponding resonance structures. Such an EVB/shift approach appears valuable for assessing the effects of different catalysts and solvents on complex chemical reactions. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 607–625, 2000     

Chemical insight into electron density and wave functions: software developments and applications to crystals, molecular complexes and materials science

This paper overviews the work made by our group during the past 10–15 years on crystalline systems, semiconductor surfaces, molecular complexes and on materials of interest for technological applications, such as the defective silicon or the novel generation thermoelectric materials. Our main aim of extracting chemical insight into the analysis of electron densities and computed wave functions is illustrated through a number of examples. The recently proposed Source Function analysis is reviewed and a few of its more interesting applications are summarized. Software package developments, motivated by the need of a direct comparison with experiment or by the help these packages can provide for interpreting complex experimental outcomes, are described and future directions outlined. A particular emphasis is given to the TOPOND and TOPXD programs, which enable one to analyze theoretical and experimental crystalline densities using the rigorous framework of the Quantum Theory of Atoms in Molecules, due to Bader.