Showing papers on "Ab initio quantum chemistry methods published in 2008"

Do Special Noncovalent π–π Stacking Interactions Really Exist?

Abstract: Noncovalent interactions play an increasingly important role in modern chemical research, and are nowadays considered as cornerstones in supramolecular chemistry, materials science, and even biochemistry. When unsaturated organic groups are involved in noncovalent interactions, the terms “p–p stacking”, or more generally “p–p interactions” are often used. As noted recently, this classification has a quite mysterious flavor. For larger structures, p–p stacking is a phenomenon that is theoretically not well understood, although some progress has been made. From many studies of the benzene dimer and other complexes involving phenyl rings, it can be concluded that the p orbitals do not function as in conventional overlapdriven covalent bonding, although this is not common knowledge. The prototypical benzene dimer is nowadays considered a typical van der Waals complex in which the long-range dispersion interactions (dominant R 6 dependence of the interaction energy on interfragment distance) play the major role. As a consequence, the dimer is unbound at uncorrelated Hartree–Fock and many density functional theory (DFT) levels. This more sophisticated view is increasingly replacing Hunter6s model of p–p interactions, which (over)emphasises the mainly quadrupole–quadrupole electrostatic component of the interaction in benzene-type systems (see Ref. [13] for recent theoretical work on polar psystems). Because van der Waals complexes are formed by almost all neutral, closed-shell molecules, which are considered exclusively herein, what should be so special about the interaction between stacked aromatic units compared to, for example, saturated (hydrogenated) rings of about the same size. This mainly energetic difference is termed herein the p–p stacking effect (PSE). For example, benzene and cyclohexane both exist as fluids at room temperature, which indicates similar intermolecular interactions. According to accurate CCSD(T) computations, the stacked (parallel-displaced, PD) benzene dimer has an even smaller binding energy than the pentane dimer ( 2.8 vs. 3.9 kcalmol ), 14] which has the same number of electrons. These observations seem to be incompatible with the assumption of special p–p interactions. On the other hand, it is known that larger polycyclic aromatic hydrocarbons (PAHs) behave differently to large alkanes; for example, PAHs become increasingly insoluble in common organic solvents with increasing size. Thus the magnitude of the intermolecular interactions and possibly also their fundamental character is more strongly size-dependent in aromatic systems than in saturated systems. The clarification of this matter, and the question as to whether the term “p–p interaction” makes sense from a theoretical point of view, is the central topic of the work presented herein. The linear condensed acenes, from benzene (number of rings n= 1) to tetracene (n= 4), and the corresponding perhydrogenated ring systems (all trans–all anti stereoisomers) were used as models. Homo-dimers of stacked (aromatic with Ci, except for the PD benzene dimer, which has C2h symmetry, and saturated with C2h symmetry) and T-shaped orientation (aromatic only, C2v) are investigated. The Tshaped forms are important in the crystal packing of aromatic molecules, as analyzed in detail by Desiraju and Gavezzotti. For saturated dimers, no well-defined T-shaped structures could be found. Energy-minimized dimer structures for n= 1 and n= 4 are shown as an example in Figure 1.

Charge transport in chemically doped 2D graphene.

Abstract: We report on a numerical study of electronic transport in chemically doped 2D graphene materials. By using ab initio calculations, a self-consistent scattering potential is derived for boron and nitrogen substitutions, and a fully quantum-mechanical Kubo-Greenwood approach is used to evaluate the resulting charge mobilities and conductivities of systems with impurity concentration ranging within [0.5, 4.0]%. Even for a doping concentration as large as 4.0%, the conduction is marginally affected by quantum interference effects, preserving therefore remarkable transport properties, even down to the zero temperature limit. As a result of the chemical doping, electron-hole mobilities and conductivities are shown to become asymmetric with respect to the Dirac point.

Investigations into the Nature of Halogen Bonding Including Symmetry Adapted Perturbation Theory Analyses.

Abstract: In recent years it has been recognized that, because of their unique properties, halogen bonds have tremendous potential in the development of new pharmaceutical compounds and materials. In this study we investigate the phenomenon of halogen bonding by carrying out ab initio calculations on the halomethane-formaldehyde complexes as well as the fluorine substituted FnH3-nCX···OCH2 dimers, where the halogen bonding halogens (X) are chlorine, bromine, and iodine. Coupled cluster (CCSD(T)/aug-cc-pVTZ) calculations indicate that the binding energies for these type of interactions lie in the range between −1.05 kcal/mol (H3CCl···OCH2) and −3.72 kcal/mol (F3CI···OCH2). One of the most important findings in this study is that, according to symmetry adapted perturbation theory (SAPT) analyses, halogen bonds are largely dependent on both electrostatic and dispersion type interactions. As the halogen atom involved in halogen bonding becomes larger the interaction strength for this type of interaction also gets large...

Compression curves of transition metals in the Mbar range: Experiments and projector augmented-wave calculations

Abstract: The ambient temperature equations of state (EoS) of iron, cobalt, nickel, zinc, molybdenum, and silver have been measured by x-ray diffraction. These transition metals were compressed using diamond anvil cells with a helium pressure transmitting medium. The maximum pressure reached during these experiments varied between 65 GPa (for cobalt) and 200 GPa (for iron). This work completes previous measurements on six other metals [Phys. Rev. B 70, 094112 (2004)] to quantify the differences between ab initio calculations and experiment on a large experimental set of transition metals. The compression curves of iron, cobalt, nickel, zinc, molybdenum, silver, platinum, and gold are also calculated ab initio within the density-functional theory (DFT) formalism using the projector augmented-wave (PAW) method and different exchange-correlation functionals (LDA, GGA-PBE, GGA-PBEsol). The difference between PAW and available all-electron calculations is found to be negligible up to very high pressures. The success of each exchange-correlation functional is correlated with the atomic number. For all metals, the bulk modulus becomes overestimated at high pressure. In addition, this extended data set of metals' EoS enables to reduce further, but marginally, the systematic uncertainty of the high-pressure metrology based on the ruby standard.

Cholesky Decomposition-Based Multiconfiguration Second-Order Perturbation Theory (CD-CASPT2): Application to the Spin-State Energetics of Co(III)(diiminato)(NPh).

Abstract: The electronic structure and low-lying electronic states of a Co-III(diiminato)(NPh) complex have been studied using mulficonfigurational wave function theory (CASSCF/CASPT2) The results have been compared to those obtained with density functional theory. The best agreement with ab initio results is obtained with a modified B3LYP functional containing a reduced amount (15%) of Hartree-Fock exchange. A relativistic basis set with 869 functions has been employed in the most extensive ab initio calculations, where a Cholesky decomposition technique was used to overcome problems arising from the large size of the two-electron integral matrix. It is shown that this approximation reproduces results obtained with the full integral set to a high accuracy, thus opening the possibility to use this approach to perform multiconfigurational wave-function-based quantum chemistry on much larger systems relative to what has been possible until now.

Rotationally invariant ab initio evaluation of Coulomb and exchange parameters for DFT+U calculations

Abstract: Conventional density functional theory (DFT) fails for strongly correlated electron systems due to large intra-atomic self-interaction errors. The DFT+U method provides a means of overcoming these errors through the use of a parametrized potential that employs an exact treatment of quantum mechanical exchange interactions. The parameters that enter into this potential correspond to the spherically averaged intra-atomic Coulomb (U) and exchange (J) interactions. Recently, we developed an ab initio approach for evaluating these parameters on the basis of unrestricted Hartree-Fock (UHF) theory, which has the advantage of being free of self-interaction errors and does not require experimental input [Mosey and Carter, Phys. Rev. B 76, 155123 (2007)]. In this work, we build on that method to develop a more robust and convenient ab initio approach for evaluating U and J. The new technique employs a relationship between U and J and the Coulomb and exchange integrals evaluated using the entire set of UHF molecular orbitals (MOs) for the system. Employing the entire set of UHF MOs renders the method rotationally invariant and eliminates the difficulty in selecting unambiguously the MOs that correspond to localized states. These aspects overcome two significant deficiencies of our earlier method. The new technique is used to evaluate U and J for Cr(2)O(3), FeO, and Fe(2)O(3). The resulting values of U-J are close to empirical estimates of this quantity for each of these materials and are also similar to results of constrained DFT calculations. DFT+U calculations using the ab initio parameters yield results that are in good agreement with experiment. As such, this method offers a means of performing accurate and fully predictive DFT+U calculations of strongly correlated electron materials.

A Photoelectron Spectroscopic and Theoretical Study of B16− and B162−: An All-Boron Naphthalene

Abstract: The structure and chemical bonding of B16− were studied using ab initio calculations and photoelectron spectroscopy. Its global minimum is found to be a quasi-planar and elongated structure (C2h). Addition of an electron to B16− resulted in a perfectly planar and closed shell B162− (D2h), which is shown to possess 10 π electrons with a π-bonding pattern similar to that of naphthalene and can thus be considered as an “all-boron naphthalene”, a new member in the growing family of hydrocarbon analogues of boron clusters.

Construction of a generalized gradient approximation by restoring the density-gradient expansion and enforcing a tight Lieb–Oxford bound

Abstract: Recently, a generalized gradient approximation (GGA) to the density functional, called PBEsol, was optimized (one parameter) against the jellium-surface exchange-correlation energies, and this, in conjunction with changing another parameter to restore the first-principles gradient expansion for exchange, was sufficient to yield accurate lattice constants of solids. Here, we construct a new GGA that has no empirical parameters, that satisfies one more exact constraint than PBEsol, and that performs 20% better for the lattice constants of 18 previously studied solids, although it does not improve on PBEsol for molecular atomization energies (a property that neither functional was designed for). The new GGA is exact through second order, and it is called the second-order generalized gradient approximation (SOGGA). The SOGGA functional also differs from other GGAs in that it enforces a tighter Lieb-Oxford bound. SOGGA and other functionals are compared to a diverse set of lattice constants, bond distances, and energetic quantities for solids and molecules (this includes the first test of the M06-L meta-GGA for solid-state properties). We find that classifying density functionals in terms of the magnitude mu of the second-order coefficient of the density gradient expansion of the exchange functional not only correlates their behavior for predicting lattice constants of solids versus their behavior for predicting small-molecule atomization energies, as pointed out by Perdew and co-workers [Phys. Rev. Lett. 100, 134606 (2008); Perdew ibid. 80, 891 (1998)], but also correlates their behavior for cohesive energies of solids, reaction barriers heights, and nonhydrogenic bond distances in small molecules.

Improving hydrogen storage capacity of MOF by functionalization of the organic linker with lithium atoms.

Abstract: A combination of quantum and classical calculations have been performed in order to investigate hydrogen storage in metal-organic frameworks (MOFs) modified by lithium alkoxide groups. Ab initio calculations showed that the interaction energies between the hydrogen molecules and this functional group are up to three times larger compared with unmodified MOF. This trend was verified by grand canonical Monte Carlo (GCMC) simulations in various thermodynamic conditions. The gravimetric capacity of the Li-modified MOFs reached the value of 10 wt % at 77 K and 100 bar, while our results are very promising at room temperature, too, with 4.5 wt %.

Ab initio molecular dynamics using hybrid density functionals

Abstract: Ab initio molecular dynamics simulations with hybrid density functionals have so far found little application due to their computational cost. In this work, an implementation of the Hartree-Fock exchange is presented that is specifically targeted at ab initio molecular dynamics simulations of medium sized systems. We demonstrate that our implementation, which is available as part of the CP2K/Quickstep program, is robust and efficient. Several prescreening techniques lead to a linear scaling cost for integral evaluation and storage. Integral compression techniques allow for in-core calculations on systems containing several thousand basis functions. The massively parallel implementation respects integral symmetry and scales up to hundreds of CPUs using a dynamic load balancing scheme. A time-reversible multiple time step scheme, exploiting the difference in computational efficiency between hybrid and local functionals, brings further time savings. With extensive simulations of liquid water, we demonstrate the ability to perform, for several tens of picoseconds, ab initio molecular dynamics based on hybrid functionals of systems in the condensed phase containing a few thousand Gaussian basis functions.

Binding of CO, NO, and O2 to heme by density functional and multireference ab initio calculations.

Abstract: Using the CASSCF/CASPT2 approach, along with several DFT methods (PBE0, B3LYP, BP86, OLYP), we have investigated the bonding of CO, NO, and O2 molecules to two model heme systems: an iron(II) porphyrin with and without an axial imidazole ligand. The experimentally available binding energies are best reproduced by the CASPT2 method and with the OLYP functional. The other functionals considered perform much worse, either severly overbinding (BP86) or underbinding (B3LYP, PBE0). Significant discrepancies between the different density functionals are observed, not only for the energetics but sometimes also for structure predictions. This confirms our viewpoint that a balanced treatment of the electronic exchange and correlation is vital to describe the weak metal−ligand bond between heme and CO, NO, or O2. The binding energies ΔEb were split into two contributions: the so-called spin-pairing energy ΔEsp and the “inherent” binding energy ΔEb0, and both contributions were analyzed in terms of method and basis s...

Quantum-interference-controlled molecular electronics.

Abstract: Quantum interference in coherent transport through single molecular rings may provide a mechanism to control the current in molecular electronics. We investigate its applicability, using a single-particle Green function method combined with ab initio electronic structure calculations. We find that the quantum interference effect (QIE) is strongly dependent on the interaction between molecular pi-states and contact sigma-states. It is masked by sigma tunneling in small molecular rings with Au leads, such as benzene, due to strong pi-sigma hybridization, while it is preserved in large rings, such as [18]annulene, which then could be used to realize quantum interference effect (QIE) transistors.

Thermophysical properties of warm dense hydrogen using quantum molecular dynamics simulations

Abstract: We study the thermophysical properties of warm dense hydrogen by using quantum molecular dynamics simulations. Results are presented for the pair distribution functions, the equation of state, and the Hugoniot curve. From the dynamic conductivity, we derive the dc electrical conductivity and the reflectivity. We compare with available experimental data and predictions of the chemical picture. In particular, we discuss the nonmetal-to-metal transition, which occurs at about 40 GPa in the dense fluid.

Density matrix renormalization group calculations on relative energies of transition metal complexes and clusters.

Abstract: The accurate first-principles calculation of relative energies of transition metal complexes and clusters is still one of the great challenges for quantum chemistry. Dense lying electronic states and near degeneracies make accurate predictions difficult, and multireference methods with large active spaces are required. Often density functional theory calculations are employed for feasibility reasons, but their actual accuracy for a given system is usually difficult to assess (also because accurate ab initio reference data are lacking). In this work we study the performance of the density matrix renormalization group algorithm for the prediction of relative energies of transition metal complexes and clusters of different spin and molecular structure. In particular, the focus is on the relative energetical order of electronic states of different spin for mononuclear complexes and on the relative energy of different isomers of dinuclear oxo-bridged copper clusters.

Application of static charge transfer within an ionic-liquid force field and its effect on structure and dynamics.

Abstract: The effects of linear scaling of the atomic charges of a reference potential on the structure, dynamics, and energetics of the ionic liquid 1,3-dimethylimidazolium chloride are investigated. Diffusion coefficients that span over four orders of magnitude are observed between the original model and a scaled model in which the ionic charges are +/-0.5 e. While the three-dimensional structure of the liquid is less affected, the partial radial distribution functions change markedly--with the positive result that for ionic charges of +/-0.7 e, an excellent agreement is observed with ab initio molecular dynamics data. Cohesive energy densities calculated from these partial-charge models are also in better agreement with those calculated from the ab initio data. We postulate that ionic-liquid models in which the ionic charges are assumed to be +/-1 e overestimate the intermolecular attractions between ions, which results in overstructuring, slow dynamics, and increased cohesive energy densities. The use of scaled-charge sets may be of benefit in the simulation of these systems--especially when looking at properties beyond liquid structure--thus providing an alternative to computationally expensive polarisable force fields.

Nitrile and thiocyanate IR probes: Quantum chemistry calculation studies and multivariate least-square fitting analysis

Abstract: Hydration effects on the CN stretching mode frequencies of MeCN and MeSCN are investigated by carrying out ab initio calculations for a number of MeCN-water and MeSCN-water complexes with varying number of water molecules. It is found that the CN frequency shift induced by the hydrogen-bonding interactions with water molecules originate from two different ways to form hydrogen bonds with the nitrogen atom of the CN group. Considering the MeCN- and MeSCN-water cluster calculation results as databases, we first examined the validity of vibrational Stark effect relationship between the CN frequency and the electric field component parallel to the CN bond and found no strong correlation between the two. However, taking into account of additional electric field vector components is a simple way to generalize the vibrational Stark theory for the nitrile chromophore. Also, the electrostatic potential calculation method has been proposed and examined in detail. It turned out that the interactions of water molecu...

Cl–π interactions in protein–ligand complexes

Abstract: During systematic analysis of nonbonded contacts in protein–ligand complexes derived from crystal structures in the Protein Data Bank, Cl–π interactions have been found, not only in the well-documented serine proteases but also, to a lesser extent, in other proteins. From geometric analysis of such Cl–π interactions in the crystal structures, two distinct geometries were found: the “edge-on” approach of a Cl atom to a ring atom or C–C bond and the “face-on” approach toward the ring centroid with an average interatomic distance of 3.6 A. High-level ab initio calculations using benzene–chlorohydrocarbon model systems elucidated that the calculated Cl–π interaction energy is −2.01 kcal/mol, and the dispersion force is the major source of attraction. We also discussed the geometric flexibility in Cl–π interactions and a relationship between the intensity of the π density in an aromatic ring and the interaction position of the Cl atom.

Intermolecular forces in an ionic liquid ([Mmim][Cl]) versus those in a typical salt (NaCl).

Abstract: Understanding chemical bonding and intermolecular forces is one of the major topics in chemistry. In general, chemical compounds are divided into classes based on their properties. The class of ionic liquids (ILs) has been known since the beginning of the last century. Owing to their tuneable properties and low vapor pressure, ILs have become a hot research area with a wide range of applications in recent years. A promising route towards better understanding ionic liquids is to determine the dominating intermolecular forces and compare them to those in an example such as NaCl, which is solid at room temperature and, in other words, a compound representative of its class. One must be careful in probing for general properties of ILs, because the search for generality has led to many myths about ionic liquids. Often properties of ionic liquids are explained by the chemical nature of the particular substance and are not a general feature. As an example for imidazolium-based ionic liquids, we chose 1,3-dimethylimidazolium chloride ([Mmim][Cl]). If one considers the theoretically predicted total interaction energies from ab initio calculations between a typical cation and an anion at the equilibrium distance in a typical IL, these energies range from 300 to 400 kJmol , in agreement with Ref. [13] (for details on the methods see the Supporting Information). Calculating the same energy for NaCl gives a value of 545.0 kJmol . This strongly points toward correlating these energies with melting points of the corresponding bulk system. It is obivous from Figure 1 that there is no correlation between the predicted energies and the melting points. A simple model for estimating the melting points of ILs suggests that most likely the liquid behavior of ILs can be attributed to large, unsymmetrical ions with high conformational flexibility. Recent studies reveal complex structures having microheterogenous polar and nonpolar domains for imidazolium-based systems with extended side chains; this phenomenon is not observed when the side chains are shorter. It has been inferred that other forces besides pure Coulombic interactions must play a role in ionic liquids. Thus, we decompose the total interaction energy of one ion pair of [Mmim][Cl] and one ion pair of NaCl by the symmetry-adapted perturbation theory (SAPT) method into different contributions in analogy to a multipole expansion (Figures 2 and 3). Note that the equilibrium distance is set to zero in order to provide comparability. For the NaCl pair (red diamonds in Figure 2) the dispersion term is negligible, whereas this contribution is comparable in magnitude to the induction term for the two conformers of the ionic liquid pair [Mmim][Cl] (blue and green diamonds in Figure 2). The main contribution to the total energy stems from the electrostatic interaction for all species, (circles in Figure 2) in agreement with Ref. [13]. For NaCl the total energy consists of only electrostatic, exchange, and induction contributions (see Figure 3; the curve with red squares almost exactly matches the curve with diamonds). In Figure 3 we can make another interesting observation concerning the minima: Whereas the NaCl pair features the minima for all curves exactly at the equilibrium distance (at zero, see black dotted vertical line), this is, surprisingly, not the case for the [Mmim][Cl] pairs (see black Figure 1. Melting points plotted against the interaction energies between one cation and one anion for several different ILs. A ball-andstick model of each IL is also given in the figure. 1: [Emim][AlCl4] , 2 : [Mmim][Cl], 3 : [Emim][BF4], 4 : [Emim][Cl], 5 : [Emim][DCA], 6 : [Emim][SCN]. Emim=1-ethyl-3-methylimidazolium ion, DCA=dicyanamide.

Nonsequential two-photon double ionization of helium

Abstract: We present accurate time-dependent ab initio calculations on fully differential and total integrated (generalized) cross sections for the nonsequential two-photon double ionization of helium at photon energies from 40 to 54 eV. Our computational method is based on the solution of the time-dependent Schr\"odinger equation and subsequent projection of the wave function onto Coulomb waves. We compare our results with other recent calculations and discuss the emerging similarities and differences. We investigate the role of electronic correlation in the representation of the two-electron continuum states, which are used to extract the ionization yields from the fully correlated final wave function. In addition, we study the influence of the pulse length and shape on the cross sections in time-dependent calculations and address convergence issues.

Full-dimensional quantum calculations of ground-state tunneling splitting of malonaldehyde using an accurate ab initio potential energy surface.

Abstract: Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1kcal∕mol, in excellent agreement with the reported ab initio value. Model one-dimensional and “exact” full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased “fixed-node” diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6cm−1 in Cartesian coordinates and 22.6cm−1 in normal coordinates, with an uncertainty of 2–3cm−1. This splitting ...

Accurate ab initio density fitting for multiconfigurational self-consistent field methods

Abstract: Using Cholesky decomposition and density fitting to approximate the electron repulsion integrals, an implementation of the complete active space self-consistent field (CASSCF) method suitable for large-scale applications is presented. Sample calculations on benzene, diaquo-tetra-mu-acetato-dicopper(II), and diuraniumendofullerene demonstrate that the Cholesky and density fitting approximations allow larger basis sets and larger systems to be treated at the CASSCF level of theory with controllable accuracy. While strict error control is an inherent property of the Cholesky approximation, errors arising from the density fitting approach are managed by using a recently proposed class of auxiliary basis sets constructed from Cholesky decomposition of the atomic electron repulsion integrals.

Monte Carlo study of the influence of antiferromagnetic exchange interactions on the phase transitions of ferromagnetic Ni-Mn-X alloys (X=In,Sn,Sb)

Abstract: On the basis of Monte Carlo simulations using the Potts model Hamiltonian, we investigate the complex temperature dependence of magnetization of ferromagnetic $\text{Ni-Mn-}X$ $(X=\text{In},\text{Sn},\text{Sb})$ Heusler alloys, in which part of the Mn atoms, which occupy the $X$ sites, interacts antiferromagnetically with the Mn atoms on the Mn sublattice. It is shown that this antiferromagnetic exchange is responsible for metamagnetic behavior and a series of magnetic phase transitions in the Heusler alloys. For an optimal choice of parameters of the model Hamiltonian, which have partially been obtained from ab initio calculations, we are able to describe the physics associated with the coupled martensitic-magnetic phase transition. The simulations lead to a qualitative agreement with the experimental magnetization curves and their dependence on temperature and composition.

A procedure for computing accurate ab initio quartic force fields: Application to HO2+ and H2O.

Abstract: A procedure for the calculation of molecular quartic force fields (QFFs) is proposed and investigated. The goal is to generate highly accurate ab initio QFFs that include many of the so-called “small” effects that are necessary to achieve high accuracy. The small effects investigated in the present study include correlation of the core electrons (core correlation), extrapolation to the one-particle basis set limit, correction for scalar relativistic contributions, correction for higher-order correlation effects, and inclusion of diffuse functions in the one-particle basis set. The procedure is flexible enough to allow for some effects to be computed directly, while others may be added as corrections. A single grid of points is used and is centered about an initial reference geometry that is designed to be as close as possible to the final ab initio equilibrium structure (with all effects included). It is shown that the least-squares fit of the QFF is not compromised by the added corrections, and the balan...

B2(BO)22- Diboronyl Diborene: A Linear Molecule with a Triple Boron−Boron Bond

Abstract: We have produced and investigated an unique boron oxide cluster, B4O2-, using photoelectron spectroscopy and ab initio calculations. Relatively simple and highly vibrationally resolved PES spectra were obtained at two photon energies (355 and 193 nm). The electron affinity of neutral B4O2 was measured to be 3.160 ± 0.015 eV. Two excited states were observed for B4O2 at excitation energies of 0.48 and 0.83 eV above the ground state. Three vibrational modes were resolved in the 355 nm spectrum for the ground state of B4O2 with frequencies of 350 ± 40, 1530 ± 30, and 2040 ± 30 cm-1. Ab initio calculations showed that neutral B4O2 (D∞h, 3Σg-) and anionic B4O2- (D∞h, 2Πu) both possess highly stable linear structures (O⋮BBBB⋮O), which can be viewed as a B2 dimer bonded to two terminal boronyl groups. The lowest nonlinear structures are at least 1.5 eV higher in energy. The calculated electron detachment energies from the linear B4O2- and the vibrational frequencies agree well with the experimental results. The ...

Cation-specific interactions with carboxylate in amino acid and acetate aqueous solutions: X-ray absorption and ab initio calculations.

Abstract: Relative interaction strengths between cations (X = Li (+), Na (+), K (+), NH 4 (+)) and anionic carboxylate groups of acetate and glycine in aqueous solution are determined. These model systems mimic ion pairing of biologically relevant cations with negatively charged groups at protein surfaces. With oxygen 1s X-ray absorption spectroscopy, we can distinguish between spectral contributions from H 2O and carboxylate, which allows us to probe the electronic structure changes of the atomic site of the carboxylate group being closest to the countercation. From the intensity variations of the COO (-) aq O 1s X-ray absorption peak, which quantitatively correlate with the change in the local partial density of states from the carboxylic site, interactions are found to decrease in the sequence Na (+) > Li (+) > K (+) > NH 4 (+). This ordering, as well as the observed bidental nature of the -COO (-) aq and X (+) aq interaction, is supported by combined ab initio and molecular dynamics calculations.

Ab initio pair potential energy curve for the argon atom pair and thermophysical properties of the dilute argon gas. I. Argon–argon interatomic potential and rovibrational spectra

Abstract: A neon–neon interatomic potential energy curve was derived from quantum-mechanical ab initio calculations using basis sets of up to t-aug-cc-pV6Z quality supplemented with bond functions and ab initio methods up to CCSDT(Q). In addition, corrections for relativistic effects were determined. An analytical potential function was fitted to the ab initio values and utilised to calculate the rovibrational spectra. The quality of the interatomic potential function was tested by comparison of the calculated spectra with experimental ones and those derived from other potentials of the literature. In a following paper the new interatomic potential is applied in the framework of the quantum-statistical mechanics and of the corresponding kinetic theory to determine selected thermophysical properties of neon governed by two-body and three-body interactions.

Ab Initio Many-Body Calculations Of n-3H, n-4He, p-3,4He, And n-10Be Scattering

Abstract: We develop a new ab initio many-body approach capable of describing simultaneously both bound and scattering states in light nuclei, by combining the resonating-group method with the use of realistic interactions, and a microscopic and consistent description of the nucleon clusters. This approach preserves translational symmetry and Pauli principle. We present phase shifts for neutron scattering on {sup 3}H, {sup 4}He and {sup 10}Be and proton scattering on {sup 3,4}He, using realistic nucleon-nucleon potentials. Our A = 4 scattering results are compared to earlier ab initio calculations. We demonstrate that a proper treatment of the coupling to the n-{sup 10}Be continuum is essential to explain the parity-inverted ground state in {sup 11}Be.