Interaction of Methane with Single-Walled Carbon Nanotubes： Role of Defects,Curvature and Nanotubes Type

We investigate the interaction of single-walled carbon nanotubes （SWCNTs） and methane molecule from the first principles. Adsorption energies are calculated, and methane affinities for the typical semiconducting and metallic nanotubes are compared. We also discuss role of the structural defects and nanotube curvature on the adsorption capability of the SWCNTs. We could observe larger adsorption energies for the metallic CNTs in comparison with the semiconducting CNTs. The obtained results for the zig zag nanotubes with various diameters reveal that the adsorption energy is higher for nanotubes with larger diameters. For defected tubes the adsorption energies are calculated for various configurations such as methane molecule approaching to the defect sites pentagon, hexagon, and heptagon in the tube surface. The results show that the introduce defects have an important contribution to the adsorption mechanism of the methane on SWNTs.     

The correcting method for the estimation of correlation energies of MF2 (M = Be,Mg, Ca) set molecules

The intrapair and interpair correlation energies of MF₂ (M = Be, Mg, Ca) set molecules are calculated and analysed, and the transferability of inner core correlation effects of M^δ+ are investigated. A detailed analysis of the comparison of correlation energies of neutral atoms with their corresponding ions of M^δ+ and F^δ/2 is given in terms of the correlation contribution of this component. The study reveals that the total correlation energy of MF₂ molecules can be obtained by summing the correlation contributions of M^δ+ and two Fδ-/2 components. This simple estimation method does shed light on the importance of searching useful means for the calculation of electron correlation energy for large biological systems.     

Electron pairs,localized orbitals and electron correlation

Presuming zeroth-order electronic wavefunctions generated from localized SCF or FORS molecular orbitals, the correlation energy is expressed as a bilinear form in terms of the pair populations of these orbitals and the projections of a correlation operator onto these orbitals. The latter are determined by fitting the correlation energies of large sets of organic molecules, which are reproduced with a mean absolute deviation of 1–3 kcal mol^?1. The resulting formula provides a ‘back-of-the-envelope’ method for estimating correlation energies and furnishes an analysis of these energies in terms of physical concepts and chemical structure. It Predicts the correlation energy of diamond (per carbon atom) to within 6 kcal mol^?1.     

On the application of a modified Sanderson formalism to atomic charge–C 1s binding energy correlation in some aromatic molecules

The modified Sanderson formalism for calculation of atomic charge in organic molecules has been shown to yield good correlation with core level binding energy (BE) shifts in organic molecules where final state relaxation effects can be neglected. Based on the concept of stability ratios, which is similar to electronegativity, this approach has the advantage of being intuitive in addition to being computationally non-intensive. However, in aromatic molecules where delocalized π electrons can contribute significantly to final-state relaxation energies, no attempt has been made to study whether such a correlation is feasible. In this communication, we seek to study the correlation between the C 1s binding energy (BE) in some aromatic organic molecules and the atomic charge on the carbons determined using the modified Sanderson method. A linear regression curve is found to fit the data satisfactorily with the degree of fit being better than for charges calculated by the MNDO quantum chemical method. There is, however, a difference in the regression curves for aromatic molecules and molecules with carbon in the sp³-hybridized state (Sastry, J. Electron Spectrosc., in press). If the discrepancy is attributed to final-state core hole relaxation effects, calculated relaxation energies are found to be unphysical. This aspect notwithstanding, the quality of the regression found for aromatic molecules suggests that the modified Sanderson formalism can be applied to aromatic molecules with some care.     

Electronics and Structural Properties of Single-Walled Carbon Nanotubes Interacting with a Glucose Molecule：ab initio Calculations

The adsorption of glucose molecule on single-walled carbon nanotubes (SWCNTs) is investigated by density functional theory calculations. Adsorption energies and equilibrium distances are evaluated, and glucose binding to the typical semiconducting and metallic nanotubes with various diameters and chirality are compared. We also investigated the role of the structural defects on the adsorption capability of the SWCNTs. We could observe larger adsorption energies for the larger diameters semiconducting CNTs, while the story is paradoxical for the metallic CNTs. The obtained results reveal that the adsorption energy is significantly higher for nanotubes with higher chiral angles. Finally, the adsorption energies are calculated for defected nanotubes for various configurations such as glucose molecule approaching to the pentagon, hexagon, and heptagon sites in the tube surface. We find that the respected defects have a minor contribution to the adsorption mechanism of the glucose on SWNTs. The calculation of electron transfers and the density of states supports that the electronic properties of SWCNTs do not change significantly after the gluycose molecular adsorption. Consequently, one can predict that presence of glucose would neither modify the electronic structure of the SWCNTs nor direct to a change in the conductivity of the intrinsic nanotubes.     

Electronics and Structural Properties of Single-Walled Carbon Nanotubes Interacting with a Glucose Molecule: ab initio Calculations

The adsorption of glucose molecule on single-walled carbon nanotubes(SWCNTs)is investigated by density functional theory calculations.Adsorption energies and equilibrium distances are evaluated,and glucose binding to the typical semiconducting and metallic nanotubes with various diameters and chirality are compared.We also investigated the role of the structural defects on the adsorption capability of the SWCNTs.We could observe larger adsorption energies for the larger diameters semiconducting CNTs,while the story is paradoxical for the metallic CNTs.The obtained results reveal that the adsorption energy is significantly higher for nanotubes with higher chiral angles.Finally,the adsorption energies are calculated for defected nanotubes for various configurations such as glucose molecule approaching to the pentagon,hexagon,and heptagon sites in the tube surface.We find that the respected defects have a minor contribution to the adsorption mechanism of the glucose on SWNTs.The calculation of electron transfers and the density of states supports that the electronic properties of SWCNTs do not change significantly after the gluycose molecular adsorption.Consequently,one can predict that presence of glucose would neither modify the electronic structure of the SWCNTs nor direct to a change in the conductivity of the intrinsic nanotubes.     

Dynamic correlation

From a knowledge of the Hartree-Fock and exact non-relativistic energies of atoms, the correlation energy E_c, as defined by Lowdin, may be calculated. For atoms this correlation is defined as dynamic correlation. The separate like-spin and unlike-spin contributions, E_cσσ, E_cαβ, may be calculated as a sum of pair energies from quantum chemistry; we have used the unrestricted M?ller-Plesset second-order algorithm, and then scaled them to give E_c. These three values may also be computed using dynamic correlation functionals, with the Stoll partitioning. The VWN, LYP and P91 functionals were studied for the atoms from H to Ar. Although the total correlation energies of LYP and P91 are similar, only P91 gives a semi-sensible breakdown into the E_cσσ, and E_cαβ components. It is immediately apparent that a new functional, OPTC, derived from the P91 components as 0.6625 x E_cσσ, + 1. 1015 x E_cαβ is an improvement (its mean absolute error is only 0.006 E_h). Using the recently introduced improved exchange functional OPTX (obtained through a fit to the HF energies of atoms), Kohn-Sham calculations were performed on the atoms using the OPT(=OPTX + OPTC) functional. The total energies have a mean absolute error of 0.006 E_h. The study then moves to molecules. First it is shown that the dynamic correlation energy contribution to the dissociation energies is very similar (within 2kcalmol^?1 in most cases), whether it is calculated with LYP, P91 or OPTC. A calculation is then made of the HF contribution, the dynamic contribution through OPTC and the left-right contribution through OPTX, to molecular binding. In many cases the sum agrees with the observed value, but in some cases the prediction is significantly in error, e.g. O₂ is overbound by 10 kcal mol^?1. Thus either OPTX or OPTC or both are inadequate. An attempt was made to determine improved local exchange and correlation functionals by fitting to both atomic and molecular data, but this was unsuccessful. The conclusion is that the method is close to the limit of accuracy achievable from separately optimized local exchange and correlation functionals. Finally, a new hybrid functional O3LYP, which is a substantial improvement on B3LYP, is presented.     

Long range C,N and H atom-atom potential parameters from ab initio dispersion energies for different azabenzene dimers

After comparing theoretically the atom-atom model with the multipole expansion for the dispersion energy, we derive atom-atom potential parameters for C, N and H contacts by fitting the atom-atom dispersion energies to the dispersion interactions calculated by an ab initio method for the following (aza)-benzene molecules: benzene, pyridine, pyridazine, pyrimidine, pyrazine, s-triazine and s-tetrazine. The data base for the fit consists of 1200 randomly chosen configurations for each azabenzene dimer. The optimized parameters for the carbon and hydrogen contacts in particular are not unique ; reasonably good fits are obtained with different parameter sets. A very simple rule, which relates the atom-atom potentials to the isotropic molecular C₆ dispersion coefficients, already leads to good estimates for the parameters. From the empirical parameter sets available in the literature the William-Govers set yields results which are close to our ab initio dispersion energies.     

Interaction Energies in a System of Three Organic Molecules Versus Their Structure and Mutual Induction

A method for the analysis of the interaction energies in a three-molecule nanocluster containing hydrocarbon molecules with double carbon–carbon bonds is presented. It is assumed that one of the molecules (pentene) has a dipole moment; the other two (aromatic molecules) have no dipole moments. The molecules in the considered three-molecule nanocluster are bounded by the long-range dispersion and induction interactions with the Coulomb repulsive forces at short intermolecular distances taken into account. Analytical expressions are obtained in terms of the presented method to calculate the dispersion and induction energies. In these expressions, the Coulomb repulsion at short distances is taken into account for each pair of molecules, which is possible owing to the specific charge properties of the double bonds in the considered molecules, which lead to the residual positive charges at the carbon atoms in these bonds. It was shown that the total interaction energy reached its minimum at smaller intermolecular distances between each pair of molecules compared with those in the corresponding isolated two-molecular nanocluster consisting of the same molecules.     

Equilibrium molecular energies used to obtain molecular dissociation energies and heats of formation within the bond-order correlation approach

Ab initio calculations including electron correlation are still extremely costly, except for the smallest atoms and molecules. Therefore, our purpose in the present study is to employ a bond-order correlation approach to obtain, via equilibrium molecular energies, molecular dissociation energies and heats of formation for some 20 molecules containing C, H, and O atoms, with a maximum number of electrons of around 40. Finally, it is important in the proposed procedure to include electron correlation effects in the basis set choice when determining thermodynamic properties. With the optimum choice of basis set, the average percentage error for some 20 molecules is approximately 20% for heats of formation. For molecular dissociation energies the average error is much smaller: ～0.4%.     

Atomic additivity of the correlation energy in molecules—an ab initio MP4 and G3 study

It is shown that the closed shell valence electron molecular correlation energy of organic molecules in their ground states is a homogeneous multilinear function of the numbers of neutral atoms in their canonical hybridization state. The additivity is a robust feature, which holds for MP2(fc), MP3(fc) and MP4(fc) model calculations. The latter results obtained on a test set of 91 widely different organic molecules, exhibiting a whole gamut of electronic structure patterns, are excellent as evidenced by the absolute average deviation from the additivity values (AAD) of only 1.4 kcal mol^?1 and R ² = 0.999 93. The maximum absolute deviation (MAD) is 5.3 kcal mol^?1. The additivity formula for the total molecular electron correlation retrieved from G3 calculations also has an excellent performance (AAD = 1.2 kcal mol^?1, R ² = 0.999 98 and MAD = 7.2 kcal mol^?1). If it is taken into account that the additivity formulae require only back of the envelope calculations, these results are remarkable indeed, in particular since the G3 correlation energies span a very large range from 180.7 (methane) to 1642.8 (hexafluorocyclopropane) kcal mol^?1. Comparison of the exact electron correlation energies in free atoms with the corresponding average correlation energies in molecules reveals that a substantial increase in the latter provides an important contribution in overcoming a very strong Coulomb repulsion between the nuclei. It is shown that the additivity formulae are useful in detecting some special molecular features such as strong resonance and anti-aromaticity.     

A Theoretical Study of a Single-Walled ZnO Nanotube as a Sensor for H_2 O Molecules

We have studied the property of single-walled ZnO nanotubes with adsorbed water molecules, and theoretically designed a new sensor for detecting water molecules using single-walled ZnO nanotubes using a combination of density functional theory and the non-equilibrium Green's function method. Details of the geometric structures and adsorption energies of the H 2 O molecules on the ZnO nanotube surface have been investigated. Our computational results demonstrate that the formation of hydrogen bonding between the H 2 O molecules and the ZnO nanotube, and adsorption energies of the H 2 O molecules on the ZnO nanotube are larger than the adsorption energies of other gas molecules present in the atmospheric environment. Moreover, the current-voltage curves of the ZnO nanotube with and without H 2 O molecules adsorbed on its surface are calculated, the results of which showed that the H 2 O molecules form stable adsorption configurations that could lead to the decrease in current. These results suggest that the single-walled ZnO nanotubes are able to detect and monitor the presence of H 2 O molecules by applying bias voltages.     

Left-right correlation energy

We first attempt to determine a local exchange functional E_x[p] which accurately reproduces the Hartree-Fock (HF) energies of the 18 first and second row atoms. E_x[p is determined from p and |δp|, and we find that we can improve significantly upon Becke's original generalized gradient approximation functional (commonly called B88X) by allowing the coefficient of the Dirac exchange term to be optimized (it is argued that molecules do not behave like the uniform electron gas). We call this new two parameter exchange functional OPTX. We find that neither δ p or t = Σ δ _i |² improve the fit to these atomic energies. These exchange functionals include not only exchange, but also left-right correlation. It is therefore proposed that this functional provides a definition for exchange energy plus left-right correlation energy when used in Kohn-Sham (KS) calculations. We call this energy the Kohn-Sham exchange (or KSX) energy. It is shown that for nearly all molecules studied these KSX energies are lower than the corresponding HF energies, thus giving values for the non-dynamic correlation energy. At stretched geometries, the KSX energies are always lower than the HF energies, and often substantially so. Furthermore all bond lengths from the KSX calculations are longer than HF bond lengths and experimental bond lengths, which again demonstrates the inclusion of left-right correlation effects in the functional. For these reasons we prefer to split the correlation energy into two parts: left-right correlation energy and dynamic correlation energy, arguing that the usage of the words ‘non-dynamic’ or ‘static’ or ‘near-degeneracy’ is less meaningful. We recognize that this definition of KSX is not precise, because the definition of a local E_x[p] can never be precise. We also recognize that these ideas are not new, but we think that their importance has been insufficiently recognized in functional determination. When we include third row atoms in our analysis, we are unable to find a local exchange functional which is a substantial improvement over B88X for the reproduction of HF energies. This must arise from the effects of the core orbitals, and therefore we do not consider that this detracts from the improved accuracy of OPTX. We report some MCSCF calculations constructed from bonding-antibonding configurations, from which we attempt to calculate ab initio left-right correlation. There is only moderate agreement between the two approaches. Finally we combine the OPTX functional with established correlation functionals (LYP, P86, P91) to form OLYP, OP86 and OP91; OLYP is a great improvement on BLYP for both energy and structure, and OP86, OP91 are an improvement over BP86, BP91 for structure. The importance of the exchange functional for molecular structure is therefore underlined.     

Total electron ionization cross-sections for molecules of astrochemical interest

ABSTRACT

We describe a newly upgraded instrument for measuring absolute total electron ionization cross-sections over the energy range from 0 to 300?eV, and present cross-sections for nine previously unstudied molecules, as well as several small molecules for which comparison data is available. The measured cross-sections are compared with the predictions of the BEB model, and show reasonable agreement with the model, albeit peaking at higher electron energies than predicted by the model. We show that the maxima in the cross-sections follow an additivity model, such that the molecular cross-sections can be expressed as a sum over contributions from the constituent atoms. These contributions have been determined from a global fit to the data for all molecules studied, and allow maximum cross-sections to be predicted for molecules that have not been studied to date. We demonstrate the expected correlation between the maximum ionization cross-section and the molecular polarisability, and show that the atomic contributions to the cross-section show a similar dependence on the atomic polarisability. The observed correlation can be used as an alternative method for predicting unknown maximum cross-sections.     

A Theoretical Study of a Single-Walled ZnO Nanotube as a Sensor for H2O Molecules

We have studied the property of single-walled ZnO nanotubes with adsorbed water molecules, and theoretically designed a new sensor for detecting water molecules using single-walled ZnO nanotubes using a combination of density functional theory and the non-equilibrium Green's function method. Details of the geometric structures and adsorption energies of the H₂O molecules on the ZnO nanotube surface have been investigated. Our computational results demonstrate that the formation of hydrogen bonding between the H₂O molecules and the ZnO nanotube, and adsorption energies of the H₂O molecules on the ZnO nanotube are larger than the adsorption energies of other gas molecules present in the atmospheric environment. Moreover, the current-voltage curves of the ZnO nanotube with and without H₂O molecules adsorbed on its surface are calculated, the results of which showed that the H₂O molecules form stable adsorption configurations that could lead to the decrease in current. These results suggest that the single-walled ZnO nanotubes are able to detect and monitor the presence of H₂O molecules by applying bias voltages.     

水分子对碳链的输运性质影响的第一性原理研究

用基于密度泛函理论的非平衡格林函数方法研究了水分子对7个碳原子组成的一维原子链的输运性质的影响.碳原子链放在具有有限截面的Al(100)电极中.研究发现，碳原子链上的水分子的数目和放置的位置的不同将对体系输运性质产生很大的影响.特别是，单个H₂O分子对碳链平衡电导的影响随其摆放位置的不同而出现奇偶振荡，例如，当位于奇数编号的碳原子上时，电导取极大值，当位于偶数编号的碳原子上时，取极小值.将两个H₂O分子置于不同的碳原子正上方时，在不同的位置平衡电导相差很大，在某些特殊的情况下原本受到抑止的第三个本征通道也有较大的贡献.此外，还研究了放置两个水分子时，体系的电流-电压(I-V)特性，随着水分子的数目和放置的位置不同，某些情况可能出现较大幅度的负微分电阻，而在另一些情况下却没有出现. 关键词： 平衡电导 透射谱 负微分电阻     

Aun(n=2,3,4)团簇与乙醇分子相互作用的第一性原理研究

利用密度泛函理论研究了Au_n(n=2,3,4)团簇与乙醇分子的吸附机理.研究结果表明:Au_n(n=2,3,4)团簇分别能够吸附1到n个乙醇分子,生成Au_n-(C₂H₆O)_1-n配合物;Au₄团簇吸附乙醇分子有多种构型,通过分析吸附能和Mulliken电荷分布,确定了4个乙醇分子的吸附顺序及相应的稳定构型;Au_n(n=3,4)团簇在吸附最后一个乙醇分子时改变了前面Au—O成键的作用模式,而是选择Au—H成键;作为吸附主体的金团簇和被吸附的乙醇分子在吸附前后构型变化都很少,它们之间的吸附作用为弱相互作用. 关键词： 金团簇 乙醇分子 密度泛函理论     

Beyond the random-phase approximation for the electron correlation energy: the importance of single excitations

The random-phase approximation (RPA) for the electron correlation energy, combined with the exact-exchange (EX) energy, represents the state-of-the-art exchange-correlation functional within density-functional theory. However, the standard RPA practice--evaluating both the EX and the RPA correlation energies using Kohn-Sham (KS) orbitals from local or semilocal exchange-correlation functionals--leads to a systematic underbinding of molecules and solids. Here we demonstrate that this behavior can be corrected by adding a "single excitation" contribution, so far not included in the standard RPA scheme. A similar improvement can also be achieved by replacing the non-self-consistent EX total energy by the corresponding self-consistent Hartree-Fock total energy, while retaining the RPA correlation energy evaluated using KS orbitals. Both schemes achieve chemical accuracy for a standard benchmark set of noncovalent intermolecular interactions.     

Δ(3, 3) excitations and effective three-nucleon forces in finite nuclei

Contributions from three-body terms with intermediate Δ(3,3) isobar excitations to the ground state energies of nuclei are investigated. These terms can either be understood as three-body clusters in a many-body theory including isobar excitations explicitly or as contributions to an effective three-nucleon force. For the example ¹⁶O the resulting contribution is attractive and its value is typically about ?0.5 MeV per nucleon. This is smaller than the typical values of 1 MeV per nucleon repulsion obtained from the modifications of the effective two-body nucleon-nucleon interaction in nuclei due to intermediate Δ(3, 3) configurations. The gain in energy from the three-body terms including Δ(3,3) configurations, however, is of the same importance as the contribution from three-body terms including nucleons only.     

Calculation of the structure of carbon clusters based on fullerene-like C<Subscript>24</Subscript> and C<Subscript>48</Subscript> molecules

Equilibrium structures obtained by linking with valence bonds the carbon carcasses of two fullerene-like molecules have been studied by molecular dynamics simulation. In free fullerene, carbon atoms form sp² hybridized bonds, but at places of links between fullerenes, sp³ hybridized bonds are formed, which determines the changes in the properties of such structures. In the literature, the topology of diamond-like phases is described, but equilibrium clusters based on fullerene-like molecules are underexplored. The right angles between the C–C bonds are energetically unfavorable, and the reduction in the energy of clusters in the process of relaxation is connected with the optimization of valence angles, which leads to a reduction in the symmetry of clusters and, in a number of cases, even to disruption of some valence bonds. It is shown that different fashions of linking two fullerenes result in the formation of clusters with different structures and energies. Different initial conditions can lead to different configurations of clusters with the same topology. Among the analyzed clusters, a structure with the minimum potential energy per atom was found. The results of this work contribute to the study of the real structure of carbon clusters.