Complete Neglect Of Differential Overlap Research Articles

Electron Emission Cross Section from Methane under 250 keV Proton Impact

We measure double differential cross sections (DDCS) of electrons emitted from CH4 molecules in collisions with 250 keV protons. The projectile ions are obtained from a 400 kV electron cyclotron resonance-based ion accelerator (ECRIA). We study the energy and angular distributions of the electron DDCS. The observed double and single differential and the total cross section are compared with the state-of-the-art continuum distorted wave eikonal initial state (CDW-EIS) model predictions. Two different approaches are used considering the different target descriptions: complete neglect of differential overlap (CNDO) and molecular orbital (MO) approximations. The MO model uses two different scaling parameters (d = 0.7 and 1.0). In the energy distribution of the DDCS, the carbon KLL Auger line is also observed at 240 eV. The single differential cross section (SDCS) and total cross section (TCS) are derived. Both the MO-based CDW-EIS models are in good agreement with the experimental results; however, the CNDO approach overestimates the data.

Single-Capture Cross Sections from Biological Molecules and Noble Gases by Bare Ion Impact

In this work, we report theoretical electron capture cross sections for single-electron removal from molecules of biological interest and noble gases by bare ion (H+ and He2+) impact at energies ranging from 25 to 10,000 keV/amu. We use a distorted wave (DW) method where the intermediate continuum state of the active electron with the target ion has been taken into account. This method is developed within the framework of the independent electron model taking particular care of the representation of the bound continuum target states. Two different approximations have been considered for molecular targets: molecular representation of the bound-state target wavefunction and Bragg’s additivity rule. The molecular orbital for targets are described within the framework of the complete neglect of differential overlap (CNDO) method based on the linear combination of atomic orbital (LCAO) approximation. Using the DW method, we have also calculated the K-, L- and M- shell electron capture in collisions of bare ions with three noble gases He, Ne, and Ar respectively. Contributions from different molecular orbitals and different shells to the total cross sections (TCS) are studied. The preference of electron capture occurs in accordance as the binding energy of the active electron in molecular orbital and atomic shell. The maximum contributions to TCS for SC comes from the less bound electrons in repetitive orbitals, whereas the tightly bound electrons dominate the TCS at higher projectile energy regime. Variation of TCS with impact energy are compared with the available experimental observation and other theoretical findings. We find that the present theoretical method is satisfactory in both intermediate and high-energy region for molecules as well as noble gas targets to give reliable outcomes compared to other theoretical methods.

Structural and optical properties of novel nematogens based on DFT and semiempirical method—A comparative picture

ABSTRACTA comparative picture of structural, and optical properties of nematogens, viz., bis(4-propyloxyphenyl) 1,12-dicarba-closo-dodecaborane-1,12-dicarboxylate (Nematic1), and bis(4-butoxyphenyl) 1,10-dicarba-closo-decaborane-1,10-dicarboxylate (Nematic2) have been studied in ultraviolet (UV) and visible (Vis) regions. The structure of nematogens have been optimized using the Density functional B3LYP with 6–31+G (d) basis set using crystallographic geometry as input. The electronic structure of the molecules has been evaluated using the DFT, and semiempirical methods, namely; CNDO/S (complete neglect of differential overlap/ spectroscopy) and INDO/S (intermediate neglect of differential overlap/ spectroscopy). The HOMO (Highest Occupied Molecular Orbital)/LUMO (Lowest Unoccupied Molecular Orbital) energies, and oscillator strength (f) have also been reported using these methods. The electronic absorption spectra of the molecules have been simulated by employing the DFT method, semiempirical parameterizations. Molecular charge distribution and phase stability of these nematogens have been analyzed based on Mulliken and Loewdin population analysis. It has been observed that Nematic2 molecule shows the much flexibility for electronic transitions over a long wavelength region that leads to high photosensitivity.

Validation and application of modeling algorithms for the design of molecularly imprinted polymers.

In the study, four different semiempirical algorithms, modified neglect of diatomic overlap, a reparameterization of Austin Model 1, complete neglect of differential overlap and typed neglect of differential overlap, have been applied for the energy optimization of template, monomer, and template-monomer complexes of imprinted polymers. For phosmet-, estrone-, and metolcarb-imprinted polymers, the binding energies of template-monomer complexes were calculated and the docking configures were assessed in different molar ratio of template/monomer. It was found that two algorithms were not suitable for calculating the binding energy in template-monomers complex system. For the other algorithms, the obtained optimum molar ratio of template and monomers were consistent with the experimental results. Therefore, two algorithms have been selected and applied for the preparation of enrofloxacin-imprinted polymers. Meanwhile using a different molar ratio of template and monomer, we prepared imprinted polymers and nonimprinted polymers, and evaluated the adsorption to template. It was verified that the experimental results were in good agreement with the modeling results. As a result, the semiempirical algorithm had certain feasibility in designing the preparation of imprinted polymers.

Electronic excitations of C60 aggregates

Excitation properties of the isolated C(60) and (C(60))(N) model clusters (N = 2, 3, 4, 6 and 13) are studied using an a priori parameterized and self-consistent Hamiltonian, the Complete Neglect of Differential Overlap considering the l azimuthal quantum number method. This method properly describes electron excitations of the isolated C(60) after the configuration interaction of singles (CIS) procedure, when those are compared with experimental data in n-hexane solution and in a molecular beam. Geometry models of (C(60))(N) clusters to model the effect of aggregation were obtained from the fullerene fcc crystal. Some peaks in the low energy edge of the absorption spectrum appear corresponding to clustering effects, as well as small increases of bandwidths in the strong bands at the UV region. An analysis of the theoretical absorption spectrum for dimer models has been carried out, taking into account the influence of the distance between fullerene centers. The density of states of CIS for fullerene clusters in the range from 2.0 to 6.5 eV shows the possibility of electron transitions as functions of the size of the clusters.

Ab initio large unit cell calculations of the electronic structure of diamond nanocrystals

In order to reduce computational efforts, and separate surface and core properties, diamond nanocrystals in the present model is represented by a heterojunction between the surface and the core in which the surface represents the outer most four layers and the core by the rest of the internal region of nanocrystal. Ab initio restricted Hartree-Fock (RHF) method coupled with the large unit cell method (LUC) is used to determine the electronic structure and physical properties of diamond nanocrystals core part with different sizes. The use of STO-3G basis choice is made to be able to compare to semiempirical methods using the complete neglect of differential overlap (CNDO) that uses Slater type orbitals (STO). The oxygenated (001)-(1 × 1) facet that expands with larger sizes of nanocrystals is also investigated to determine the rule of the surface in nanocrystals electronic structure. The results show that the present method agrees with semiempirical method contraction of lattice constant with increasing nanocrystal size but disagrees with energy gap variation with nanocrystal size in some regions. After nearly 1.4 nm the energy gap which is controlled by surface states begins to rise. The lowest unoccupied molecular orbital (LUMO) is attributed to surface states that largely reduce the value of energy gap. The sources of disagreement between semiempirical and ab initio results are discussed. The present method shows a maximum increment of the lattice constant by 3.3% over the calculated bulk for the smallest diamond nanocrystals. The surface states are found mostly non-degenerated because of the effect of surface discontinuity and oxygen atoms. Valance and conduction bands are wider on the surface due to splitting and oxygen atoms. The method also shows fluctuations in the converged energy gap, valence band width and cohesive energy of the core part of nanocrystal. These fluctuations might partially explain the controversial experimental results for diamond nanocrystals greater than 1.4 nm in size. The method of the present model has threefold results; it can be used to obtain the electronic structure of bulk, surface, and nanocrystals.

Computer simulation of molecular organization in a nematogen. The role of thermodynamic parameters

Computer simulations of molecular organization in a nematogen, alkenyl bicyclohexylnitrile (ALBCHNL), have been carried out with respect to translational and orientational motions. The atomic net charge and dipole moment at each atomic center has been evaluated using the complete neglect of differential overlap (CNDO/2) method. The modified Rayleigh-Schroedinger perturbation theory, along with multicentered-multipole expansion method, has been employed to evaluate long-range intermolecular interactions, while a “6-exp” potential function has been assumed for short-range interactions. The total interaction energy values obtained through these computations have been used as input to calculate the thermodynamical parameters such as entropy and Helmholtz free energy of each configuration at room temperature (300 K), nematic-isotropic transition (364.7 K) and above transition temperature (450 K). An attempt has been made to understand the molecular organization, to develop a new and interesting model for nematogen based on the thermodynamic parameters introduced in this article.

Approximate quantum mechanical method for describing excitations and related properties of finite single-walled carbon nanotubes

Optical properties of three kinds of zigzag (5,0), (13,0), and (9,0) single-walled carbon nanotubes (SWCNTs) are studied using an approximate quantum mechanical method named complete neglect of differential overlap, which distinguishes basis atomic orbitals with different azimuthal $l$ quantum numbers (CNDOL). This method models the electron energy transitions and excited state charge distributions through a configuration interaction of singly (CIS) excited determinants allowing the direct understanding of properties related with the total electronic wave function of nanoscopic systems, projecting a reliable quantum mechanical understanding to real life objects. The finite SWCNT's structures were obtained by replicating the unit cells of periodic SWCNTs and saturating the edge dangling bonds with hydrogens. The unit cell was previously relaxed using standard density functional theory methods. The behavior of these SWCNTs were interpreted in the framework of the CNDOL scheme by increasing the lengths of the tubes above 3 nm. As the nanotubes grow in length, the position of excited states for each SWCNT evolve differently: in contrast with (9,0) SWCNT, which exhibits favorable conditions for photoexcitation, the (13,0) and (5,0) SWCNTs do not show a lowering of the lowest excited states. This behavior is discussed by taking into account electron---electron interactions as considered in the framework of the CIS procedure. Furthermore, the (13,0) and (5,0) SWCNTs present forbidden transitions for the lowest excitations and its first dipole-allowed transitions are at 0.9--1.0 and 1.4--1.6 eV, respectively. In contrast, (9,0) SWCNT allows excitations by photon at less than 0.4 eV as the length of the nanotube tends to infinite. Excitons appear more bounded, energetically and spatially, in the (13,0) than in the (9,0) and (5,0) SWCNTs.

Electronic Structure of AlP Under Pressure Using Semiempirical Method

The effect of pressure on the structural and electronic parameters of zinc-blende aluminum phosphide crystal has been investigated using the large unit cell within the framework of complete neglect of differential overlap and the linear combination of atomic orbital approximation. Cohesive energy, indirect band gap, valence bandwidth, conduction bandwidth, bulk modulus, and valence charge distribution are all obtained. The calculations show a good agreement of lattice constant, cohesive energy, valence bandwidth, and bulk modulus with the experimental data. The calculated band gap is twice the experimental value. This high value of the band gap is expected in Hartree-Fock method. The effect of pressure on the aforementioned properties is investigated. It is found that the indirect band gap, valence bandwidth, bulk modulus and cohesive energy increase with increasing pressure, while the conduction bandwidth decreases. The maximum value of pressure is taken to be 9 GPa, because beyond this value, the phase of AlP transforms from zinc blende phase to nickel arsenic phase.

Structure-activity correlations for phenoxyacetic acids and indoleacetic acids used for plant growth regulation

Experiments on substituted indole-3-acetic acid and halogenated indoles have led to the suggestion that fractional positive charge development on the—NH group of the indole ring at a fortuitous distance (5.5 Å) from the carboxylic acid function is requisite for plant auxin activity exhibited by indoleacetic acids [W. L. Porter and K. V. Thimann, Phytochem. 4, 229 (1965)]. Calculations using self-consistent field (scf) molecular orbital (mo) theory with complete neglect of differential overlap (cndo) were carried out on a series of halo-substituted phenoxyacetic and indoleacetic acids. No net positive charge development calculated for the N—H group in the substituted indoleacetic acids gave the observed order of auxin activity. No development of positive charge in the ∏-pz system calculated for N was in any way comparable to the observed auxin activity increase. However, the energies of the lowest unoccupied molecular orbitals (lumo) decreased nearly linearly with increasing auxin activity. Twelve substituted phenoxyacetic acids, also treated, showed a correlation between the lumo energies and the square of the lumo coefficients at the ortho position, respectively, and plant auxin activity when the lipophilicity of the molecules in the series was taken into account. The correlation coefficient of the activity as function of lumo energy was virtually identical to that calculated from the data of Hansch and coworkers [see J. Amer. Chem. Soc. 85, 2817 (1963)], using ring substituent Hammett sigma values for the ortho position instead of the lumo energies.

Electron-intramolecular vibration coupling in TTF-TCNQ systems

We report calculations of the linear electron molecular vibration coupling constants in TCNQ, TCNQ−, and TTF. A complete neglect of differential overlap (cndo) type parameterization is used for the electronic structure and a modified valence force field (mvff) approach for the determination of the vibrational normal modes. Direct experimental information on the magnitude of the coupling constants can be obtained in the gaseous phase from photoemission experiments and in the solid state from polarized optical reflectance measurements on organic linear chain conductors. Agreement between theory and experiment is good. The relevance of electron-intramolecular vibrational coupling constants for the stabilization of the charge density wave (cdw) state in TCNQ anion radical salts and in the determination of polaron binding energies is stressed.

Size effects of semiempirical large unit cell method in comparison with nanoclusters properties of diamond-structured covalent semiconductors

Self-consistent Hartree–Fock method within the framework of large unit cell (LUC) formalism using complete neglect of differential overlap (CNDO) is used to simulate size effects on nanoclusters of covalent crystalline diamond-structured semiconductor C, Si, Ge, and Sn using k=0 approximation. Three sizes are investigated namely 8, 64, and 216 atom LUCs. Cohesive energy, energy gap, valence band width, and hybridization orbitals are obtained from electronic structure calculations. Charge distribution, density of states, and orbital wave functions are also reported. Sensitivity analysis of the dependence of some of the calculated properties on model parameters is performed. Results revealed that electronic properties converge to some limit as the size of the LUC increases and that the 216 atoms LUC is very near to the bulk of these materials. Increasing LUC size or atomic number of covalent semiconductor removed some electronic cloud from bonding region to the spherical region around the atom. The same is true in going to higher energy levels. Increasing the size of LUC also resulted in a decrease of energy gap, increasing valance band width, and generally slightly increasing the cohesive energy. The model predicts energy gap reduction in going from ultra-small nanoclusters to the bulk of the four elements to be around 2 eV. The approximations imposed on the present model calculations leading to resemblance with nanoclusters properties in addition to the evidences of such resemblance such as orbital shapes and trends of data are discussed. Small nanoclusters are expected to have stronger directional bonds than in their bulk structure. The smallest cells (8 atoms) are found to be of a slightly longer lattice constant of the order 0.01, 0.1, 0.04, and 0.02 a.u. for diamond, silicon, germanium, and tin, respectively. X-ray form factors show a slight decrease of low-angle lines when we go to larger cells that was also found experimentally. To the best of my knowledge, this is the first systematic determination of covalent nanoclusters lattice constant and X-ray form factors variation with ultra-small nanocluster size.

Some CNDO Calculations and Structural Spectroscopic Studies of Paracetamol Complexes

Through complete neglect of differential overlap (CNDO) calculations of the electronic energy among different possible structures of paracetamol (PA) molecule, it has been concluded that its structure has Cs point group symmetry of the cis‐form in which the methyl group has a restricted free rotation around its bond with the carbon atom of the amide group. The electronic spectra of PA compound were studied in different polar and nonpolar solvents. The temperature effect on the electronic spectra confirms the presence of one conformer only. The hydrogen bonding and the orientation energies of the polar solvents were determined from the studies of mixed solvents. Complexes of PA with metal ions M(II) (Cu++, Zn++, or Fe++) of ratio 2:1, respectively, were prepared, and their structure has been confirmed by elemental analysis, atomic absorption spectra, IR spectra, and 1H‐NMR spectra. It has been concluded that the structure of the complexes has C2h point group symmetry in which two PA molecules are chelated to any one of the metal ions Cu++, Zn++, and Fe++.

A self-consistent transport model for molecular conduction based on extended Hückel theory with full three-dimensional electrostatics

We present a transport model for molecular conduction involving an extended Hückel theoretical treatment of the molecular chemistry combined with a nonequilibrium Green's function treatment of quantum transport. The self-consistent potential is approximated by CNDO (complete neglect of differential overlap) method and the electrostatic effects of metallic leads (bias and image charges) are included through a three-dimensional finite element method. This allows us to capture spatial details of the electrostatic potential profile, including effects of charging, screening, and complicated electrode configurations employing only a single adjustable parameter to locate the Fermi energy. As this model is based on semiempirical methods it is computationally inexpensive and flexible compared to ab initio models, yet at the same time it is able to capture salient qualitative features as well as several relevant quantitative details of transport. We apply our model to investigate recent experimental data on alkane dithiol molecules obtained in a nanopore setup. We also present a comparison study of single molecule transistors and identify electronic properties that control their performance.

Statistical atomic models with complete neglect of differential overlap for the study of free and confined systems

AbstractTotal ground‐state energies of free and confined many‐electron atoms are studied through use of the Thomas–Fermi–Dirac–Weizsäcker energy density functional using known properties of the orbital electron densities. The concept of complete neglect of differential overlap put forward by Pople and Segal is combined with the original ideas of Wang and Parr on the development of statistical atomic models. For free atoms, it is found that total energies within 0.1% difference relative to Hartree–Fock values can be obtained using a prefactor λ = 1/8 in the Weizsäcker inhomogeneity correction. The corresponding description of total radial densities and shell structure is found to be overall moderate when compared with Hartree–Fock calculations. It is also shown, for the first time, that this approach properly accounts for the ground‐state energy evolution of many‐electron atoms confined by hard spherical walls. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Role of molecular geometry, valence charge distribution and vibrational modes in bioactivity of cyclodiene insecticides

Fourier transform infrared spectral measurement and complete normal mode calculations have been carried out for heptachlor and wedge modes have been identified. In addition, valence charge distributions have been obtained for aldrin, endrin, heptachlor, beta-endosulfan, cis-chlordane, trans-chlordane, photo-cis-chlordane and photo-trans-chlordane using CNDO/2 (complete neglect of differential overlap) molecular orbital calculations. A toxicity parameter has been defined as the product of the charge distribution and surface area of the corresponding active wedges, depending on the interacting structural features. The variation of this toxicity parameter is generally in order and in agreement with the reported measurements based on the corresponding order of observed LD50 values. Because of the structural features, the toxicity in these cases is a surface phenomenon rather than a volume effect as in the case of Gammexane, wherein the insecticide binds itself to membrane channels. Because the biological activity is a dynamic rather than a static phenomenon, it is expected that the normal modes involving the wedge atoms play an important role.

Screening and the quantitative π-model description of the optical spectra and polarizations of phenyl based oligomers

The long standing problem of the inability of many semiempirical models to correctly predict the polarization of the higher dipole allowed optical transitions of phenyl based π-conjugated polymers and molecules is examined and related to the issue of internal and external screening of π–π electron Coulomb interactions within the molecules. Following a review of previous theoretical and experimental work, π electron only the Complete Neglect of Differential Overlap (CNDO) model is presented which, for the first time, is able to predict accurately the energies and symmetries of all the observed optical transitions of benzene, biphenyl and trans-stilbene, up to ∼8–10 eV. In so doing, it is demonstrated that the problem with previous calculations was the noninclusion of screening from outside the π electron system itself. By fitting separately the spectra in hydrocarbon based condensed phases, in the gas phase and in solid rare gas matrices, and comparing the resulting model parameters, we show that, while the effects of screening from the environment are certainly noticeable, the most important spectral features—in particular the ordering of dipole allowed transitions—come from effective screening by the σ electrons. We find that both of these effects can be adequately accounted for within a π electron only model by using a dielectric constant and appropriate parameter renormalization.

Electronic properties of conjugated polymers studies by quantum molecular dynamics simulations

The growing interest in using organic materials, specifically polymers, as active media for optoelectronic devices demands a deep knowledge of the properties and behaviour at the molecular level. The lack of experimental studies at the molecular scale drives the attention to theoretical methods used to solve the Schrodinger equation of large clusters with suitable approximations. Here we present the structural and dynamic results obtained for individual molecules of poly(p-phenylene vinylene) (PPV) and polydiacetilene (PDA), through self-consistent molecular dynamics calculations with semi-empirical quantum chemistry at the complete neglect of differential overlap (CNDO).

Theoretical study of solvent effects of first-order hyperpolarizabilities of nitro–azobenzene dendrimers

The molecular structures of four generations of nitro–azobenzene dendrimers in the nonpolar solvent, carbon tetrachloride (dielectric constant, ϵ=2.2) and the polar solvents, chloroform ( ϵ=4.8) and acetonitrile ( ϵ=37.5), have been calculated using a molecular dynamics (MD) method. The first-order hyperpolarizabilities ( β) were calculated for the MD-calculated structures using the semi-empirical molecular orbital, CNDO/S-CI (complete neglect of differential overlap for spectroscopy–configuration interaction) level of theory. It was found that the dendrimers show larger β values in the polar solvents with larger dielectric constants, whose characteristics reproduce the experimental results. Although the experimentally observed enhancement of the second harmonic generation (SHG) activity that is greater than the number of chromophores is not sufficiently reproduced, the β values for the second, third, and fourth generations in acetonitrile solution are, respectively, 4.8, 8.0, and 21.7 times that of the first one (98×10 −30 esu) of the nitro–azobenzene dendrimers, whose tendency is in good qualitative agreement with the measurements compared to the azobenzene dendrimers without the nitro groups.

Charge-induced defects in poly-phenylene-vinylene (PPV)

We use a molecular dynamics method with semi-empirical quantum chemistry at the complete neglect of differential overlap (CNDO) level to study the charge-induced structural relaxation of poly( p-phenylene vinylene) (PPV) backbone, as a result of charge injection in light emitting diodes. Our results suggest that structural relaxation of PPV is accompanied by a local change in the electronic structure of the polymer leading to commonly known defects. The mobility of these defects along the polymer chains of different finite length was estimated from computer experiments. The charge rearrangement among the PPV atoms is also discussed.