Selection of terms for a CI wavefunction to preserve potential surface features

A cumulative selection procedure for choosing configuration functions for inclusion in CI calculations is described. The objective of the method is to obtain equal energy loss, relative to unselected calculations, for different states and different regions of the potential surface. Results obtained from calculations on the BH molecule indicate an overall advantage in comparison to the threshold selection procedure, particularly with regard to molecular geometry changes.     

A resonating broken-symmetry CI study of cationic states of phenalenyl dimeric compounds

We investigated several phenalenyl cationic dimer systems using ab initio methods. First, we examine the through-space cation dimer. The B3LYP method fails to describe the correct dissociation as in the case of usual cation dimers. On the other hand, the resonating broken symmetry configuration interaction (Res-BS CI) method based on unrestricted Hartree–Fock solutions yields the correct potential surface and correct charge distributions. Furthermore, we investigated though-bond interactions of the phenalenyl cationic dimer via acetylene and ethylene bridges. The results are considerably different from those of the though-space case, implying that these bridging units play important roles to determine charge and spin distributions. Judging from these results, different bridges-or-stacks lead to different hole behaviors, indicating the possibility of various electronic functionalities of hole-doped phenalenyl compounds. In particular, the acetylene bridge is expected to enhance the conductivity of the hole-doped phenalenyl compounds.     

A non-linear approach to configuration interaction: The low-rank CI method (LR CI)

An alternative to the linear CI expansion is presented, which is based on the spectral resolution of the double excitation CI coefficients arranged as a symmetric matrix. The resulting energy expression is a quartic function of the new variables, where the double excitation operator is given as T̂₂ = ∑_I^Mω_I(d̂_I)², where d̂d_I is a single excitation operator: d̂_I =∑_Ia,d̂^I_iaÊ_ai. Since M is a small number, the number of variables is reduced considerably compared to normal SDCI, with only little loss in accuracy. A program for non-linear CI calculations has been written and is presented with results for H₂O and N₂. The method can easily be extended to include orbital optimization and cluster terms in the wavefunction, still yielding a fully variational approximation to the Schrödinger equation.     

Modifications of virtual orbitals in the limited CI calculations for electron-rich molecules

The efficiency of modified virtual orbitals (MVO) of ionic type and of approximate orthogonalized natural orbitals (ONO) in the CI-SD calculations was studied for O₃ and SO₂ molecules and compared with the commonly used canonical virtual orbitals (CVOs). The systems studied represent a class of electron-rich molecules, in which the number of valence electron pairs exceeds substantially the number of formal chemical bonds. We found that the modified orbitals of the types studied appear to be less effective for these systems than in the similar calculations for the AH_n type molecules. Physical reasons for this difference were discussed. The evolution of spatial properties of virtual orbitals within the modification process was analyzed. © 1996 John Wiley & Sons, Inc.     

The QDPT CI approach for excitation and ionization spectra: A test on the CO molecule

A detailed analysis of the performance of the QDPT CI method, which is based on the application of quasi-degenerate perturbation theory (QDPT ) within the configuration interaction (CI ) approach, is presented by comparing the QDPT CI results and those obtained from the diagonalization of the perturbative space. Calculations of core and valence excitation and ionization spectra of the CO molecule, which is a strongly correlated system, are carried out using a DZ basis set and the accuracy of the eigenvalues and eigenfunctions is checked through the comparison of excitation and ionization energies and intensities. Excellent agreement is obtained for all values, indicating the computational advantages of QDPT CI which is able to explore wide energy ranges without requiring the diagonalization of large spaces. © 1995 John Wiley & Sons, Inc.     

Application of graphical methods of spin algebras to limited CI approaches. II. A simple open shell case

A time independent diagrammatic technique based on the Wick theorem and graphical methods of spin algebras, as outlined in Part I, is applied to a simple open shell case having one unpaired electron in addition to the pure singlet closed shell. Compact explicit expressions for the matrix elements of a spin independent Hamiltonian between conveniently chosen spin symmetry adapted states are given for the ground, mono- and bi-excited configurations.     

A multireference direct CI program based on the symmetric group graphical approach

Specific features of an SGGA-based multireference direct CI program working in large internal spaces are discussed. In particular, advantages resulting from the explicit separation of the orbital and the spin spaces are explored. Concepts allowing for the efficient creation of a flexible and symmetry-adapted CI basis, for the high-speed generation of the coupling coefficients and for structuring a simple permutation driven algorithm to handle the orbital space are briefly discussed.Alexander von Humboldt Fellow 1985/87     

The surface potential of a spherical colloid particle: functional theoretical approach

By using the iterative method in functional analysis, the potential of the electrical double layer of a spherical colloid particle, which is represented by the so-called Poisson-Boltzmann (PB) equation, has been solved analytically under general potential conditions. With the help of the diagram method in mathematics, the surface potential of the particle has been defined from the second iterative solution. The influence of the parameters included in the solutions on the surface potential has been studied. The results show that the surface potential of the particle increases as the temperature of the system, the aggregation number, and the concentration of ions increase, but decreases with an increase in the dielectric constant and the valence of the ions. The corresponding space charge density also has been illustrated in this work.     

Application of graphical methods of spin algebras to limited CI approaches. I. Closed shell case

The time independent diagrammatic technique based on the mathematical methods of quantum electrodynamics (second quantization, Wick's theorem, Feynman-like diagrams) is combined with graphical techniques of spin algebras to derive general expressions for the matrix elements of spin independent one- and two-particle operators between spin symmetry adapted ground, mono- and bi-excited configurations of a closed shell system. Two coupling schemes are considered for bi-excited states and their relative merits are discussed. Finally, the results are used to derive compact expressions for the coupling coefficients of the direct configuration interaction from molecular integrals (CIMI ) method.     

SCF CI potential energy surfaces for the HCN α HCN isomerisation reaction

We report X ¹A' and 2 ¹A' potential energy surfaces for HCN. Thermal isomerisation on the ground-state surface is discussed. An avoided crossing between X ¹A' and 2 ¹A' suggests alternative isomerisation mechanisms via this excited state. The X¹A'-2¹A' interaction may also allow the formation of CN(X ²Σ⁺) in photodissociation of XCN molecules.     

Electric potential induced dissociation of hybridized DNA with hairpin motif immobilized on silicon surface

DNA probes were immobilized on silicon surfaces through ester condensation between the -COOH group on the surface and the terminal -OH group in the oligonucleotide, and the surface density of DNA probes against the concentration of immobilization solution was measured by radioactive labeling. The dissociation of DNA duplex on the surface by an electric potential was studied with the scanning potential hairpin denaturation/dissociation (SPHD). The influence of the stem length in the hairpin probe on the SPHD curve was systematically investigated. It was found that the capability of discrimination on single nucleotide polymorphism (SNP) by a hairpin probe was related to the free energy of formation of the secondary structure in the probe (DeltaG(ss)). In our system, when DeltaG(ss) was around -3 kcal/mol, an optimal recognition of SNP was reached and the SPHD curve was sigmoid. In contrast, the equivalent SPHD curve from a linear probe was exponential-decay alike with a poor discrimination of SNP. The concentration dependent experiments showed good linearity between the melting potential and logarithm of target concentration in the range of 1 x 10(-9) to 5 x 10(-7) M.     

The surface trans effect: influence of axial ligands on the surface chemical bonds of adsorbed metalloporphyrins

The chemical bond between an adsorbed, laterally coordinated metal ion and a metal surface is affected by an additional axial ligand on the metal ion. This surface analogon of the trans effect was studied in detail using monolayers of various M(II)-tetraphenylporphyrins (MTTPs, M = Fe, Co, Zn) and their nitrosyl complexes on a Ag(111) surface. X-ray photoelectron spectroscopy (XPS) shows that the oxidation state of the Fe and Co (but not Zn) ions in the MTPP monolayers is reduced because of the interaction with the substrate. This partial reduction is accompanied by the appearance of new valence states in the UV photoelectron and scanning tunneling spectra (UPS and STS), revealing the covalent character of the ion-substrate bond. Subsequent coordination of nitric oxide (NO) to the metal ions (Fe, Co) reverses these surface-induced effects, resulting in an increase of the oxidation states and the disappearance of the new valence states. Removal of the NO ligands by thermal desorption restores the original spectroscopic features, indicating that the described processes are fully reversible. The NO coordination also changes the spin state and thus the magnetic properties of the metal ions. Density-functional theory (DFT) calculations on model systems provide structural and energetic data on the adsorbed molecules and the surface chemical bond. The calculations reveal that competition effects, similar to the trans effect, play a central role and lead to a mutual interference of the two axial ligands, NO and Ag, and their bonds to the metal center. These findings have important implications for sensor technology and catalysis using supported planar metal complexes, in which the activity of the metal center is sensitively influenced by the substrate.     

The surface potential of solvent and the intraphase pre-existing potential

Using own and literature data, the differences of real solvation energy for ferricenium and ferrocene in six solvents are found. These quantities are confronted with the calculated difference of the dielectric response energies plus nonelectrostatic energies for the redox couple. Such a comparison allows determining the sum of the surface and intraphase potentials. The comparison of these sums with the experimental values of the surface potential differences obtained by the measuring of Volta potentials allowed determining the differences of pre-existing intraphase potentials formed by solvent molecules on the ferrocene molecule. Thus, the intraphase potentials are evaluated for the first time, using an approach not based on the molecular-dynamic modeling. Using some approximations, the surface potentials of the studied solvents are found.     

The role of exchange-correlation functionals in the potential energy surface and dynamics of N(2) dissociation on W surfaces

We study the dissociative adsorption of N(2) on W(100) and W(110) by means of density functional theory and classical dynamics. Working with a full six-dimensional adiabatic potential energy surface (PES), we find that the theoretical results of the dynamical problem strongly depend on the choice of approximate exchange-correlation functional for the determination of the PES. We consider the Perdew-Wang-91 [Perdew et al., Phys. Rev. B 46, 6671 (1992)] and Perdew-Burke-Ernzerhof (RPBE) [Hammer et al., Phys. Rev. B 59, 7413 (1999)] functionals and carry out a systematic comparison between the dynamics determined by the respective PESs. Even though it has been shown in earlier works that the RPBE may provide better values for the chemisorption energies, our study brings evidence that it gives rise to a PES with excessive repulsion far from the surface.     

A spline-fitted potential surface for bent triatomic molecules

The rotated-Morse curve cubic spline method developed previously is extended to bent triatomic molecules by using 2D cubic spline. The Yates—Lester potential for bent H₃ is shown to be accurately fitted over the entire range of the three internal coordinates, with a standard deviation of less than 1 kcal mol^?1. The spline method compares favorably in computational speed with the analytical potential.     

A first-principles study of molecular oxygen dissociation at an electrode surface: a comparison of potential variation and coadsorption effects

Influences of coadsorbed sodium and water, aqueous solvent, and electrode potential on the kinetics of O(2) dissociation over Pt(111) are systematically investigated using density functional theory models of vacuum and electrochemical interfaces. Na coadsorption alters the electronic states of Pt to stabilize the reactant (O(2)*), transition, and product (2O*) states by facilitating electron donation to oxygen, causing a more exothermic reaction energy (-0.84 eV for Na and O(2), -0.81 eV for isolated O(2)) and a decrease in dissociation barrier (0.39 eV for Na and O(2), 0.57 eV for isolated O(2)). Solvation decreases the reaction energy (-0.67 eV) due to enhanced hydrogen bond stabilization of O(2)* compared to 2O*. The influence of Na is less pronounced at the solvated interface (barrier decreases by only 0.11 eV) because H(2)O screens Na charge-donation. In the electrochemical model system, the dissociation energy becomes more exothermic and the barrier decreases toward more positive potentials. Potential-dependent behavior results from changes in interfacial dipole moment and polarizability between O(2)*, the dissociation transition state, and 2O*; each are influenced by changes in adsorption and hydrogen bonding. Coadsorption of Na in the solvated system dampens the dipole moment change between O(2)* and 2O* and significantly increases the polarizability at the dissociation transition state and for 2O*; the combination causes little change in the reaction energy but reduces the activation barrier by 0.08 eV at 0 V versus NHE. The potential-dependent behavior contrasts that determined at a constant surface charge or from an applied electric field, illustrating the importance of considering the electrochemical potential at the fully-solvated interface in determining reaction energetics, even for non-redox reactions.     

Ellagitannin chemistry. Evolution of a three-component coupling strategy for the synthesis of the dimeric ellagitannin coriariin A and a dimeric gallotannin analogue

The total synthesis of the dimeric ellagitannin coriariin A is reported. The key reaction to access the dimeric framework was realized early in the synthesis pathway via a bis acylation reaction of a dehydrodigalloyl diacid with 2 equiv of a glucopyranose trichloroacetimidate. The glucose rings were subsequently functionalized, culminating in a double oxidative cyclization to form stereoselectively both (S)-HHDP ester units. This bis acylation strategy was also employed to prepare a gallotannin analogue of coriariin A whose earlier synthesis by orthoquinone dimerization was plagued by yield-limiting side reactions.     

Classical trajectory study on an ab initio CI vibrotor potential energy surface for Li+-CO differential cross sections

A previous rigid rotor potential surface for Li⁺-CO has been improved by computing surface points for two additional CO bond lengths at three different angles of orientation. The CI calculations including all single and double excitations which can be generated within the Hartree-Fock SCF molecular orbital basis have been improved by taking certain quadruple excitations into account in an approximate way. Classical trajectories computed on this surface have been used to determine differential cross sections at scattering angles of 37.1°, 43.2° and 49.2°, and for a relative kinetic energy of 4.23 eV. Comparison with experiment shows that inclusion of CO vibrations does not account for the discrepancy found previously between the classical rigid rotor and the experimental results. When summed over all final vibrational levels the vibrotor results are nearly identical to the rigid rotor cross sections.     

A note on the streaming potential method for measuring surface potentials

Due to the fact that the classical analysis of the streaming potential method for measuring surface potential is somewhat crude, the problem was analyzed by applying the fundamental principles of electrochemistry. It was found that the classical Helmholz-Smoluchowski equation is valid provided that the duct is long and narrow, in which case the potential in the bulk is uniform over the duct's cross-sectional area and varies linearly over its length.In order to follow how the surface potential of a filter varies with time, investigators have measured the streaming potential across the filter's length as well as across segments of the filter. The problem is what do such measurements mean and do the classical equations apply. It was found that the classical result is expressible in differential form from which it followed that the streaming potential over any section of the filter gives a measure of the average surface potential of that section. This is an important result because it shows that the surface potential profile of a filter can be determined from measurements of the streaming potential profile across the filter.