Computational study of glycine–(water)3 complex by density functional method

Glycine–(water)₃ complexes have been studied by means of B3LYP density functional method using 6-311++G* basis set. In the complex considered here, the three water molecule are either attached to the carboxylic group or bridge between the amino group and carboxylic group of glycine. Four such complexes are studied. Relaxation energies, two-, three- and four-body interaction energies are obtained by applying many-body analysis to know their role in binding energy of the complex. The results are compared with recent work on glycine–(water)₃ complex with group as proton donor [A. Chaudhari, P.K. Sahu, S.L. Lee, J. Chem. Phys. 120 (2004) 170]. In the most stable structure of glycine–(water)₃ complex, the three water molecules are attached to the carboxylic group of glycine and it is 5.3 kcal/mol lower in energy than that of the most stable structure reported earlier. The three-body term from water–water–water interaction in the most stable in this work and that reported earlier is unique since the distances between the water molecules are almost same. The two-body term from water–water interaction has significant contribution to the total two-body term when the distance between water molecules is less than 3 Å.     

Hydrogen-bonding interactions in acetonitrile oligomers using density functional theory method

Hydrogen-bonding interaction in acetonitrile oligomers is studied using density functional theory method. Two types of hydrogen-bonded oligomers are considered viz. cyclic and ladder. Different levels are used to optimize the geometry of acetonitrile monomer and found that at B3LYP/aug-cc-pvtz level the geometrical parameters and vibrational frequencies are in agreement with the experimental determinations. The BSSE corrected total energies of acetonitrile oligomers show that the cyclic structures are more stable than the ladder and the hydrogen bonds in former are stronger than those in the latter. Many-body analysis approach was used to study the nature of interactions between different molecules in these oligomers. It is found that the contribution from many-body energies to the binding energy of a complex is different in cyclic and ladder structures. An increase and decrease in the energy per hydrogen bond with cluster size for the cyclic and ladder structures, respectively, indicates the positive and negative hydrogen-bond cooperativity, respectively.     

An average-of-configuration method for using Kohn-Sham density functional theory in modeling ligand-field theory

The Amsterdam Density Functional (ADF) package has been used to constrain Kohn-Sham DFT in such a fashion that a transition from KS-DFT to ligand-field theory in the form of the parametrical d(q)() model is completely well-defined. A relationship is established between the strong-field approximation of the parametrical d(2) model for the tetrahedral complexes VCl(4)(-) and VBr(4)(-) and certain fixed-orbital ADF-computed energies. In this way values for all the parameters of the d(2)() model may be computed, thus allowing the ADF results to be expressed in terms of a KS-DFT energy matrix that can be diagonalized. This means that the KS-DFT deficiency with regard to computation of nondiagonal elements has been overcome and the KS-DFT eigenenergies have become available through the KS-DFT mimicking of the ligand-field plus repulsion model. By using mutually orthogonal strong-field energy matrices, the mimicking has been further elucidated. The computed values for the empirical parameters of VCl(4)(-) and VBr(4)(-) are in good agreement with experimental data. The spectrochemical and the nephelauxetic series have been computed by including the remaining halide complexes and the quantitatively special position of F(-)() among the halides corroborated for both series.     

Multicoefficient density functional theory (MC-DFT)

We have systematically tested the performance of several pure and hybrid versions of density functional methods on different types of molecular energies by combining energies calculated using more than one basis sets. Most hybrid functionals show important performance improvement as compared to methods using only a single basis set. The results suggest that, in many cases, scaling the basis set corrections is also important for density functional theory calculation. The best method, the B1B95 functional using the cc-pVDZ/cc-pVTZ/aug-cc-pVDZ basis set combination, achieves an average accuracy of 1.76 kcal/mol on a database of 109 atomization energies, 38 hydrogen-transfer barrier heights, 38 non-hydrogen-transfer barrier heights, 13 ionization potentials, and 13 electron affinities.     

Configuration interaction based on constrained density functional theory: a multireference method

Existing density functional theory (DFT) methods are typically very effective in capturing dynamic correlation, but run into difficulty treating near-degenerate systems where static correlation becomes important. In this work, we propose a configuration interaction (CI) method that allows one to use a multireference approach to treat static correlation but incorporates DFT's efficacy for the dynamic part as well. The new technique uses localized charge or spin states built by a constrained DFT approach to construct an active space in which the effective Hamiltonian matrix is built. These local configurations have significantly less static correlation compared to their delocalized counterparts and possess an essentially constant amount of self-interaction error. Thus their energies can be reliably calculated by DFT with existing functionals. Using a small number of local configurations as different references in the active space, a simple CI step is then able to recover the static correlation missing from the localized states. Practical issues of choosing configurations and adjusting constraint values are discussed, employing as examples the ground state dissociation curves of H(2) (+), H(2), and LiF. Excellent results are obtained for these curves at all interatomic distances, which is a strong indication that this method can be used to accurately describe bond breaking and forming processes.     

Block-localized wavefunction (BLW) method at the density functional theory (DFT) level

The block-localized wavefunction (BLW) approach is an ab initio valence bond (VB) method incorporating the efficiency of molecular orbital (MO) theory. It can generate the wavefunction for a resonance structure or diabatic state self-consistently by partitioning the overall electrons and primitive orbitals into several subgroups and expanding each block-localized molecular orbital in only one subspace. Although block-localized molecular orbitals in the same subspace are constrained to be orthogonal (a feature of MO theory), orbitals between different subspaces are generally nonorthogonal (a feature of VB theory). The BLW method is particularly useful in the quantification of the electron delocalization (resonance) effect within a molecule and the charge-transfer effect between molecules. In this paper, we extend the BLW method to the density functional theory (DFT) level and implement the BLW-DFT method to the quantum mechanical software GAMESS. Test applications to the pi conjugation in the planar allyl radical and ions with the basis sets of 6-31G(d), 6-31+G(d), 6-311+G(d,p), and cc-pVTZ show that the basis set dependency is insignificant. In addition, the BLW-DFT method can also be used to elucidate the nature of intermolecular interactions. Examples of pi-cation interactions and solute-solvent interactions will be presented and discussed. By expressing each diabatic state with one BLW, the BLW method can be further used to study chemical reactions and electron-transfer processes whose potential energy surfaces are typically described by two or more diabatic states.     

Molecular electronegativity in density functional theory(VIII)

Based on the density functional theory and the atom-bond electronegativity equalization model (ABEEM), a method is proposed to construct the softness matrix and to obtain the electron population normal modes (PNMs) for a closed system. Using this method the information about the bond charge polarization in a molecule can be obtained easily. The test calculation shows that the PNM obtained by this method includes all the modes about the bond charge polarization explicitly. And the bond charge polarization mode characterized by the biggest eigenvalue, which is the softest one of all modes related with chemical bonds, can describe the charge polarization process in a molecule as exquisitely as the correspondingab initio method.     

Molecular electronegativity in density functional theory (VI)

Based on the density functional theory and partitioning the molecular electron density ρ (r) into atomic electronic densities and bond electronic densities, the expressions of the total molecular energy and the “effective electronegativity” of an atom or a bond in a molecule are obtained. The atom-bond electronegativity equalization model is then proposed for the direct calculation of the total molecular energy and the charge distribution of large molecules. Practical calculations show that the atom-bond electronegativity equalization model can reproduce the correspondingab initio values of the total molecular energies and charge distributions for a series of large molecules with a very satisfactory accuracy.     

Characterization of pentacarbonyl(4-methylpyridine)chromium(0) complex using density functional theory (DFT) and Hartree-Fock (HF) computational methods

The optimized molecular structure, atomic charges, vibrational frequencies, thermodynamic properties, nuclear magnetic resonance (NMR) and ultraviolet-visible (UV-Vis) spectral data of pentacarbonyl(4-methylpyridine)chromium(0) complex have been investigated by performing ab initio Hartree-Fock (HF) and density functional theory, B3LYP, B3PW91 and BE1PBE methods with 6-311G, 6-311+G(3d,3p) and 6-31G(d,p) basis set. The calculated NMR data at 6-311G basis set, vibrational frequencies at 6-311+G(3d,3p) basis set and the optimized geometric bond lengths and bond angles at 6-31G(d,p) basis set are in good agreement with the corresponding experimental values. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have been simulated. In addition, the transition state and energy band gap and infrared intensities have also been reported.     

Real-time time-dependent density functional theory using density fitting and the continuous fast multipole method

An implementation of real-time time-dependent density functional theory (RT-TDDFT) within the TURBOMOLE program package is reported using Gaussian-type orbitals as basis functions, second and fourth order Magnus propagator, and the self-consistent field as well as the predictor–corrector time integration schemes. The Coulomb contribution to the Kohn–Sham matrix is calculated combining density fitting approximation and the continuous fast multipole method. Performance of the implementation is benchmarked for molecular systems with different sizes and dimensionalities. For linear alkane chains, the wall time for density matrix time propagation step is comparable to the Kohn-Sham (KS) matrix construction. However, for larger two- and three-dimensional molecules, with up to about 5,000 basis functions, the computational effort of RT-TDDFT calculations is dominated by the KS matrix evaluation. In addition, the maximum time step is evaluated using a set of small molecules of different polarities. The photoabsorption spectra of several molecular systems calculated using RT-TDDFT are compared to those obtained using linear response time-dependent density functional theory and coupled cluster methods.     

Conformational study of halogen- and hydrogen-substituted polythionylphosphazenes using density functional theory method

The structure and conformational stability of polythionylphosphazenes is investigated by modeling single polymer chains with small mimics. The model compounds are composed of repeat units of the corresponding polythionylphosphazenes. Two of the model compounds have hydrogens and two have chlorines as substituents on phosphorus atoms. The substituents on sulfur may be either fluorine or chlorine. Fully geometry-optimized structures and energies of the stable conformations involving rotations around the P? N bond near the sulfur are obtained using the density functional theory method. The structural and conformational analyses indicate that the rotation around the N? P bond leads to variations in the bond lengths, the SNP bond angle openings, as well as couplings between dihedral angles in different conformations in all model compounds. In addition, the conformational analysis suggests that the minima on the conformational potential energy surface in these compounds may be located in the vicinity of the following values of the NP? NS dihedral angle: -50°, 90° (or 60°), 180°, and 240°. It was found that the values of the conformational energy differences range between less than 1 to 5 kcal/mol. A comparison is made between the structural results obtained using the density functional theory and the ab initio molecular orbital theory for the global minimum structures. © 1995 John Wiley & Sons, Inc.     

A density functional theory study of the benzene-water complex

The intermolecular interaction of the benzene-water complex is calculated using real-space pseudopotential density functional theory utilizing a van der Waals density functional. Our results for the intermolecular potential energy surface clearly show a stable configuration with the water molecule standing above or below the benzene with one or both of the H atoms pointing toward the benzene plane, as predicted by previous studies. However, when the water molecule is pulled outside the perimeter of the ring, the configuration of the complex becomes unstable, with the water molecule attaching in a saddle point configuration to the rim of the benzene with its O atom adjacent to a benzene H. We find that this structural change is connected to a change in interaction from H (water)/pi cloud (benzene) to O (water)/H (benzene). We compare our results for the ground-state structure with results from experiments and quantum-chemical calculations.     

Vacancy-induced magnetism in BaTiO3(001) thin films based on density functional theory

The origin of magnetism induced by vacancies on BaTiO(3)(001) surfaces is investigated systematically by first-principles calculations within density-functional theory. The calculated results show that O vacancy is responsible for the magnetism of the BaO-terminated surface and the magnetism of the TiO(2)-terminated surface is induced by Ti vacancy. For the BaO-terminated surface, the magnetism mainly arises from the unpaired electrons that are localized in the O vacancy basin. In contrast, for the TiO(2)-terminated surface, the magnetism mainly originates from the partially occupied O-2p states of the first nearest neighbor O atoms surrounding the Ti vacancy. These results suggest the possibility of implementing magneto-electric coupling in conventional ferroelectric materials.     

Catalyzing aldehyde hydrosilylation with a molybdenum(VI) complex: a density functional theory study

[MoCl(2)O(2)] catalyzes the hydrosilylation reaction of aldehydes and ketones, as well as the reduction of other related groups, in apparent contrast to its known behavior as an oxidation catalyst. In this work, the mechanism of this reaction is studied by means of density functional theory calculations using the B3LYP functional complemented by experimental data. We found that the most favorable pathway to the first step, the Si--H activation, is a [2+2] addition to the Mo=O bond, in agreement with previous and related work. The stable intermediate that results is a distorted-square-pyramidal hydride complex. In the following step, the aldehyde approaches this species and coordinates weakly through the oxygen atom. Two alternative pathways can be envisaged: the classical reduction, in which a hydrogen atom migrates to the carbon atom to form an alkoxide, which then proceeds to generate the final silyl ether, or a concerted mechanism involving migration of a hydrogen atom to a carbon atom and of a silyl group to an oxygen atom to generate the silyl ether weakly bound to the molybdenum atom. In this Mo(VI) system, the gas-phase free energies of activation for both approaches are very similar, but if solvent effects are taken into account and HSiMe(3) is used as a source of silicon, the classical mechanism is favored. Several unexpected results led us to search for still another route, namely a radical path. The energy involved in this and the classical pathway are similar, which suggests that hydrosilylation of aldehydes and ketones catalyzed by [MoCl(2)O(2)] in acetonitrile may follow a radical pathway, in agreement with experimental results.     

Spectroscopic properties of trichlorofluoromethane CCl(3)F calculated by density functional theory

Density functional theory is used to generate local potential energy surfaces in normal coordinates for several chlorine isotopomers of trichlorofluoromethane (CCl(3)F, CFC11). An examination of predicted structures suggested that the PBE0 functional would be suitable. Anharmonic surfaces around the equilibrium geometries are reported, as determined by energies, gradients, and second derivatives. Vibrational levels for fundamentals, overtones and combination bands are reported, as well as harmonic frequencies, anharmonic constants, rotational constants, isotope shifts, and infrared intensities. These are compared with experimental information.     

Theoretical investigation of novel silsesquioxane-supported Phillips-type catalyst by density functional theory (DFT) method

In spite of great commercial importance of the Phillips CrO_x/SiO₂ catalyst and long term research efforts, the precise physicochemical nature of active sites and polymerization mechanisms still remains unclear. The difficulties in a clear mechanistic understanding of this catalyst mainly come from the complexity of the surface chemistry of the amorphous silica gel support. In this work, novel silsesquioxane-supported Phillips Cr catalysts are utilized as realistic models of the industrial catalyst for theoretical investigation using the density functional theory (DFT) method in order to elucidate the effects of surface chemistry of silica gel in terms of supporting of chromium compounds and fluorination of the silica surface on the catalytic properties of the Phillips catalyst. Both qualitative and quantitative aspects with respect to various electronic properties and thermodynamic characteristics of the model catalysts were achieved. The future prospects of a state-of-the-art catalyst design and mechanistic approaches for the heterogeneous SiO₂-supported Phillips catalyst has been demonstrated. The text was submitted by the authors in English.     

Vibrational spectroscopic and density functional theory studies of chloranil-imidazole interaction

The present work reports vibrational spectra and density functional theory calculations for chloranil, imidazole and their complexes. The experimentally observed infrared and Raman bands have been assigned with the help of calculated vibrational frequencies and potential energy distribution analysis. Some bands of chloranil and imidazole have been found to shift on the complex formation due to partial electronic charge transfer from imidazole to chloranil. The charge transfer between these molecules is also corroborated by the electronic absorption spectroscopy and calculations. The theoretical values of the interaction energy of various possible chloranil-imidazole interactions suggest that the two molecules interact preferably via N and H atoms of imidazole and CO group of chloranil with their molecular planes almost perpendicular to each other.     

Modeling of the hydration sphere of gadolinium(III) ion using density functional theory

The hydrated gadolinium(III) ion cluster Gd³⁺ (H₂O)_x, with x = 8,9, was studied using density functional theory. The different conformations of the first hydration shell were calculated. For x = 8, the results for the cubic conformation correspond to previously published Hartree-Fock and MP2 results, whereas much lower energies were found for the square antiprismatic and dodecahedral conformations. For x = 9, the energy of the tricapped trigonal prism is nearly identical to the one of the antiprism plus an extra free water molecule. © 1997 John Wiley & Sons, Inc.     

Absorption spectra of caffeic acid, caffeate and their 1:1 complex with Al(III): density functional theory and time-dependent density functional theory investigations

The UV-visible absorption spectra of caffeic acid, caffeate and of the predominant complex obtained in the presence of aluminum ion (1:1 stoichiometry) have been simulated by using the time-dependent density functional theory (TD-DFT) technique, taking into account solvent effects. Whereas the use of the B3LYP hybrid XC functional with the 6-31+G(d,p) basis set allows us to reproduce fairly well the essential features of the experimental spectra of caffeic acid and caffeate, it is necessary to introduce an effective core potential to properly describe the aluminum ion and its environment and to obtain a good agreement between theoretical and experimental spectra of the 1:1 complex. The ligand presents two potential complexing sites in competition. The results of our calculations show that the aluminum ion coordinates preferentially at the level of the catecholate group, and the [Al(H(2)O)(4)(CA)], [Al(H(2)O)(3)(OH)(CA)](-) and [Al(H(2)O)(4)(HCA)](+) complexed forms could coexist in aqueous solution at pH = 5.