Paired-permanent approach to valence bond theory

A new function called paired-permanent is defined and widely discussed, and a practicable procedure for evaluations of paired-permanents is proposed, which is similar to the Laplace method for determinants. Using the concept of paired-permanents, an efficient algorithm is presented for evaluating the Hamiltonian and overlap matrix elements in the spin-free form of valence bond (VB) theory. With the new algorithm, a spin-free wavefunction is simply written as a paired-permanent, and an overlap matrix element may be obtained by evaluating a corresponding paired-permanent. Meanwhile, the Hamiltonian matrix element is expressed in terms of the summation of the products of electronic integrals and the corresponding sub-paired-permanents     

Strength of Intramolecular Hydrogen Bonds

The concept of resonance-assisted hydrogen bonds(RAHBs)highlights the synergistic interplay between theπ-resonance and hydrogen bonding interactions.This concept has been well-accepted in academia and is widely used in practice.However,it has been argued that the seemingly enhanced intramolecular hydrogen bonding(IMHB)in unsaturated compounds may simply be a result of the constraints imposed by theσ-skeleton framework.Thus,it is crucial to estimate the strength of IMHBs.In this work,we used two approaches to probe the resonance effect and estimate the strength of the IMHBs in the two exemplary cases of the enol forms of acetylacetone and o-hydroxyacetophenone.One approach is the block-localized wavefunction(BLW)method,which is a variant of the ab initio valence bond(VB)theory.Using this approach,it is possible to derive the geometries and energetics with resonance shut down.The other approach is Edmiston’s truncated localized molecular orbital(TLMO)technique,which monitors the energy changes by removing the delocalization tails from localized molecular orbitals.The integrated BLW and TLMO studies confirmed that the hydrogen bonding in these two molecules is indeed enhanced byπ-resonance,and that this enhancement is not a result ofσconstraints.     

Optimization of specific cell-peptide interactions at the bio-material interface via regulation of nonspecific biofouling interactions

Materials that interact with biological systems in a wellcontrolled,predictable manner are highly sought after for numerous biomedical and biotechnological applications.For example,it is essential that devices such as stents or heart valves in the blood stream of patients do not cause thrombosis or infection,or that biosensors do not lose their function due to adverse interactions with the bio environment.A general approach to the design of biointerfaces that addresses this need has been pursued over a number of years[1].It has two elements:(1)prevention of non-specific interfacial interactions,and(2)promotion of specific biointeractions required for the intended functioning of the material.In practice this means preventing all interactions except for the desired one(s).With respect to the first point it should be appreciated that biological species such as proteins and cells have a natural affinity for the interfacial state.Therefore,the realization of a surface that eliminates the adsorption/adhesion of these species is extremely difficult.Grafting of PEO/PEG,other hydrophilic polymers(PVP,PHEMA),and zwitterionic polymers[2]to the surface is the most common strategy to achieve“bio-resistance”.Regarding the second point,the surface must be designed so that it interacts selectively and exclusively with the component(s)(proteins,cells)that will give the desired bioactivity.For this purpose,“ligands”that bind these specific components are incorporated into the surface[3].     

THE PARAMETER KERNELS FOR THE LINEAR INVERSION OF THE FREE OSCILLATION OF THE EARTH (Ⅱ)

This is the second instalment of the paper, in which the method of derivation of the for-mulas of parameter kernels for the spheroidal oscillations are considered. The principle ofRayleigh is used to derive the formulas of the kernels of the two elastic parameters, i.e. μand λ. But for that of ρ, it is found very difficult to get satisfactory results. A new butsimpler approach is followed and very satisfactory results are obtained.     

Popularising Chemistry in Secondary Schools——through the development of Scientific and Technological Literacy for All

While chemistry is popular for children of primary school age,its popularity drops dramatically in secondary schools,A potential solution to address this is to recognise the need for greater relevance-relevance to the curriculum,to the needs of society and,in the eyes of the students.The paper tries to puts forward a rethink of the philosophy for school chemistry education,based on the underlying belief that chemistry education is an integral part of education.The target is labelled scientific and technological literacy for all (STL),STL rejects the notion that chemistry is taught solely to acquire tha abilities to be a scientist and supports the vision of a single curriculum goal,applicable to all students.Besides being a philosophy,STL is also a teaching approach.This approach is to initiate the teaching of chemistry topics,starting from a carefully chosen society perspective and to introduce the conceptual learning on a need-to-know basis.A limitatioin is the professional development of teachers,Suggestion for overcoming this involve workshop on materials development.Aconcern is expressed in cases where teachers make use of ready-made teaching materials(a common practice),without first gaining an understanding and an appreciation of the intended philosophy related to these materials.     

Sequence pattern Correlation of Amino Acid in Collision—induced Dissociation Electrospray Ionization Mass Spectrometry

A novel approach of seuqence pattern correlation has been applied to predict an expected amino acid sequence from CID ESI-MS spectra.The proposed approach deduces sequence patterns with no help form known protein database such that it is useful to identify an unknown petide or new protein.The algorithm applies a cross-correlation to match an experimental CID spectrum with predicted sequence pattern generated form fragmentation information.The fragmentation knowledge of both y-series and other non y-series are utilized to generate the predicted sequence patterns.In contrast to the normal de novo approach,the proposed approach is insensitive to mass tolerance and non-sussceptive to spectral integrality with no need for selection of a starting point.     

THE COOLING RATE DUE TO 9.6μm OZONE BAND——A NEW APPROXIMATION

The cooling rate due to the 9.6μm band of ozone has been calculated by a new approxi-mation. Comparison with the results of the exact line-by-line integration shows that usingthis now approach makes it possible to accurately deal with integrations over wavenumber andover zenith angle and inhomogeneities of the atmosphere which have been left unsolved satis-factorily. This procedure is remarkably fast. It is also shown that the Curtis-Godson ap-proximation and the diffuse factor of 1.667 used in the usual band model techniques areinadequate for the 9.6μm ozone band.     

STABILITY AND EQUATIONS OF AMPLITUDE AND PHASE FOR DISTURBANCES ON ZONAL FLOW IN BAROTROPIC ATMOSPHERE ON SPHERICAL COORDINATES

Using the method of linear disturbances, it is proved that the sufficient and necessarycondition for neutral disturbances is that the isolines of the phase for disturbances are dis-tributed in the meridional direction, in an ideal and inviscid and nondivergent atmosphere onrotating spherical coordinates with a shear zonal parallel flow and the zonal rigid boundaryconditions. From the above conclusion, we can get that the disturbances with tilted treughmay not be neutral, and it must be amplifying or damping. Furthermore, we have derivedthe nonlinear equations of amplitude and phase for disturbances.     

气相色谱法测定线虫中的脂肪酸含量

ematode is a small organism and the traditional purification method needs a lot of nematodes , but it is difficult for us to foster many nematodes.The fatty acids in nematode were esterified directly in our analytical method,so that,the error is reduced the and sensitivity is improved. The results showed that thisimproved analysis method could be used for rapid and satisfactory determination of the fatty acid change during storagr of this group of nematodes.     

Theoretical study on stability and nonlinear optical properties of tetrahydropyrrole diradical and its isoelectronic systems in different electronic states

The polarizabilities and hyperpolarizabilities of the tetrahydropyrrole diradical in different electronic states have been investigated using ab initio and density functional theory(DFT) methods combined with the finite field(FF) approach.The polarizability average value αs is a maximum for the singlet state,while that for the closed-shell is a minimum.The trend in second hyperpolarizability average value γ is in good agreement with that for αs.The γ values of the singlet and triplet states are,respectively...     

Paired-permanent approach for VB theory (II)

Paired-permanent approach for VB theory is extensively developed. Canonical expansion of a paired-permanent is deduced. Furthermore, it is shown that a paired-permanent may be expressed in terms of the products of sub-paired-permanents of any given order and their corresponding minors. Anab initio spin-free valence bond program, called Xiamen, is implemented by using paired-permanent approach. Test calculation shows that Xiamen package is more efficient than some other programs based on the traditional VB algorithm, and it provides a new practical tool for quantum chemistry.     

Density embedded VB/MM: a hybrid ab initio VB/MM with electrostatic embedding

A hybrid QM/MM method that combines ab initio valence-bond (VB) with molecular mechanics (MM) is presented. The method utilizes the ab initio VB approach to describe the reactive fragments and MM to describe the environment thus allows VB calculations of reactions in large biological systems. The method, termed density embedded VB/MM (DE-VB/MM), is an extension of the recently developed VB/MM method. It involves calculation of the electrostatic interaction between the reactive fragments and their environment using the electrostatic embedding scheme. Namely, the electrostatic interactions are represented as one-electron integrals in the ab initio VB Hamiltonian, hence taking into account the wave function polarization of the reactive fragments due to the environment. Moreover, the assumptions that were utilized in an earlier version of the method, VB/MM, to formulate the electrostatic interactions effect on the off-diagonal matrix elements are no longer required in the DE-VB/MM methodology. Using DE-VB/MM, one can calculate, in addition to the adiabatic ground state reaction profile, the energy of the diabatic VB configurations as well as the VB state correlation diagram for the reaction. The abilities of the method are exemplified on the identity SN2 reaction of a chloride anion with methyl chloride in aqueous solution. Both the VB configurations diagram and the state correlation diagram are presented. The results are shown to be in very good agreement with both experimental and other computational data, suggesting that DE-VB/MM is a proper method for application to different reactivity problems in biological systems.     

Graphical linking of a MO multicenter bond index to VB structures

The multicenter bond index proposed within the MO framework depends on the order of the centers for which it is calculated, outside from the 3-c case. For the 6-c case, the eventually different values are 60. A graphical approach links the MO values to VB structures. Benzene, chosen as our paradigm for the 6-c case, illustrates our proposition.     

酚醛型吸附树脂对VB_(12)的吸附性能研究

研究了酚醛型吸附树脂JDW 1、JDW 2 (自制 )和DuoliteS 76 1对VB12 的静态和动态吸附 .结果表明 ,JDW 1对VB12 的吸附量达 84mg g ,明显优于DuoliteS 76 1;吸附VB12 的初始阶段 ,即达到 4 3%～6 9%平衡吸附时 ,吸附速率数据和半经验速率方程很吻合 ;酚醛型吸附树脂等温吸附VB12 的平衡吸附数据符合Langmuir方程 ,相关系数在 0 99以上 ,因此 ,酚醛型吸附树脂吸附VB12 属单分子层吸附 ;在动态条件下 ,用含甲醇 80 %溶液以 1 1BV h来洗脱吸附VB12 的JDW 2 ,在 4 2、6 4个床体积的洗脱率分别是 92 2 0 %、95 93% ,这表明酚醛型吸附树脂具有良好的洗脱性能 ,用含甲醇为 80 %溶液作洗脱剂从JDW 2洗脱VB12 ,效果很好     

VB/MM--the validity of the underlying approximations

Two recently developed methods, VB/MM and density embedded VB/MM (DE-VB/MM), are compared, and their respective approximations are examined. The two methods combine valence-bond (VB) calculations with molecular mechanics (MM) and aim to allow VB analysis of reactions in large biological environments. Furthermore, the two methods utilize two major approximations regarding both the overlap and the reduced resonance integral between the various VB configurations. The difference between the two methods, however, is that VB/MM employs these approximations for the overall interaction of the reacting fragments with their surrounding, whereas DE-VB/MM employs the approximations only with regards to the van der Waals (VdW) interactions whereas the electrostatic interactions are calculated rigorously at the quantum level. The approximations that lay the grounds for the two methods involve the assumption that the overlap between the VB configurations and the respective reduced resonance integral are both invariant to the environment. Similar approximations are utilized in several other VB-based QM/MM methods. However, although extensively used, these approximations were never rigorously proved. Here, we exploit the development of the DE-VB/MM method to numerically examine the approximations by calculating the accurate as well as the approximated values of overlap and reduced resonance integral for systems where the environment involves only electrostatic interactions. The quality of the approximations is examined together with their effect on the absolute energies, the wave function, and the overall energetics. Three test cases are chosen, the dissociation of CH 3F and LiF and the identity S N2 reaction. It is shown that the approximations are usually good with the exception of cases where extreme changes are expected in the wave function. Furthermore, the impact of the approximations on the overall wave function and the overall energetics is found to be quite small. It is concluded that VB/MM, where the approximations are used more extensively, can serve as the first method of choice.     

VB2000: Pushing valence bond theory to new limits

Full valence bond (VB) calculations for a system of N electrons have always been hindered by the rapidly growing value of N!, which effectively imposes a limit N < 20. Often, however, not all electrons in a molecule are of interest; if we focus on a “group” G of N_G electrons (e.g., in an “active” region), then it is N_G! that sets the limit. In this work, group function (GF) theory is used to represent a molecule as a collection of interacting electron groups, each with a many‐electron wave function of any chosen form (e.g., VB, MO‐SCF, MCSCF), and each GF is optimized individually in a step‐by‐step process. An efficient VB algorithm allows for up to 14 electrons in any VB group and this combination of GF and VB methods greatly extends the range of feasibility of molecular calculations with VB‐type wave functions: Thus, (1) a large system can be divided into any number of smaller subsystems (groups); (2) each group may contain any chosen number of electrons; (3) the form of any group function (including its level of accuracy) may be chosen at will by the program user. A number of sample calculations are briefly presented. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Hybrid ab initio VB/MM method--a valence bond ride through classical landscapes

The development of a new hybrid (QM/MM) method, where the QM part is treated by ab initio valence bond (VB) theory is presented. This VB/MM method has the advantages of empirical VB (EVB) methodology but does not rely on empirical parameterization for the quantum part. The method implements embedding of the quantum region of each diabatic state separately, by treating the electrostatic interactions between QM and MM regions classically. Additionally, it assumes that changes of the off diagonal matrix element due to different environments are such that the overall resonance integral does not change. These assumptions are discussed in detail and the validity of the method is shown to be successful in three different bond dissociation processes, where bond dissociation as well as solvation energies compare very well with the experimental data.     

An analysis of VB structures in MO wave functions I

Summary A new method for the resolution of an antisymmetrized product of molecular orbitals into VB structures is proposed. Here VB structures are projected from a single Slater determinant associated with the ground state using the first-order density matrix. The present theory is applied to the ground state of some conjugated hydrocarbons, and special attention is paid to the covalent type VB structures.     

The Valence-Bond (VB) Model and Its Intimate Relationship to the Symmetric or Permutation Group

VB and molecular orbital (MO) models are normally distinguished by the fact the first looks at molecules as a collection of atoms held together by chemical bonds while the latter adopts the view that each molecule should be regarded as an independent entity built up of electrons and nuclei and characterized by its molecular structure. Nevertheless, there is a much more fundamental difference between these two models which is only revealed when the symmetries of the many-electron Hamiltonian are fully taken into account: while the VB and MO wave functions exhibit the point-group symmetry, whenever present in the many-electron Hamiltonian, only VB wave functions exhibit the permutation symmetry, which is always present in the many-electron Hamiltonian. Practically all the conflicts among the practitioners of the two models can be traced down to the lack of permutation symmetry in the MO wave functions. Moreover, when examined from the permutation group perspective, it becomes clear that the concepts introduced by Pauling to deal with molecules can be equally applied to the study of the atomic structure. In other words, as strange as it may sound, VB can be extended to the study of atoms and, therefore, is a much more general model than MO.