Volume 139, Issue 20, 28 November 2013

We present a density functional theory study of the oxidation of 1D periodic rods supported along the [001] direction on the rutile TiO2(110) surface. The study shows evidence for an oxidation of the interface between the supported Au and the TiO2 crystal. The added O atoms adsorb at the 5fTi atoms in the through under the Au rod and are stabilized by charge transfer from the nearest Au atoms. Despite an extensive search, we find no low energy barrier pathways for CO oxidation involving CO adsorbed on Au and O at the perimeter of the Au/TiO2 interface. This is in part attributed the weak adsorption of CO on cationic Au at the perimeter.
 COMMUNICATIONS


Communication: Thermodynamic analysis of critical conditions of polymer adsorption
View Description Hide DescriptionPolymer adsorption to solid surfaces is a ubiquitous phenomenon, which has attracted longlasting attention. Dependent on the competition between the polymersolid adsorption and polymersolvent solvation interactions, a chain may assume either 3d solvated conformation when adsorption is weak or 2d adsorbed conformation when adsorption is strong. The transition between these conformations occurring upon variation of adsorption strength is quite sharp, and in the limit of “infinite” chain length, can be treated as a critical phenomenon. We suggest a novel thermodynamic definition of the critical conditions of polymer adsorption from the equality of incremental chemical potentials of adsorbed and free chains. We show with the example of freely jointed LennardJones chains tethered to an adsorbing surface that this new definition provides a link between thermodynamic and geometrical features of adsorbed chains and is in line with classical scaling relationships for the fraction of adsorbed monomers, chain radii of gyration, and free energy.

Communication: Benzene dimer—The free energy landscape
View Description Hide DescriptionEstablishing the relative orientation of the two benzene molecules in the dimer has remained an enigmatic challenge. Consensus has narrowed the choice of structures to either a Tshape, that may be tilted, or a parallel displaced arrangement, but the relatively small energy differences makes identifying the global minimum difficult. Here we report an ab initio CarParrinello Molecular Dynamics based metadynamics computation of the freeenergy landscape of the benzene dimer. Our calculations show that although competing structures may be isoenergetic, free energy always favors a tilted Tshape geometry at all temperatures where the bound benzene dimer exist.

Communication: An accurate global potential energy surface for the ground electronic state of ozone
View Description Hide DescriptionWe report a new fulldimensional and global potential energy surface (PES) for the O + O2 → O3 ozone forming reaction based on explicitly correlated multireference configuration interaction (MRCIF12) data. It extends our previous [R. Dawes, P. Lolur, J. Ma, and H. Guo, J. Chem. Phys.135, 081102 (2011)] dynamically weighted multistate MRCI calculations of the asymptotic region which showed the widely found submerged reef along the minimum energy path to be the spurious result of an avoided crossing with an excited state. A spinorbit correction was added and the PES tends asymptotically to the recently developed longrange electrostatic model of Lepers et al. [J. Chem. Phys.137, 234305 (2012)]. This PES features: (1) excellent equilibrium structural parameters, (2) good agreement with experimental vibrational levels, (3) accurate dissociation energy, and (4) mostnotably, a transition region without a spurious reef. The new PES is expected to allow insight into the still unresolved issues surrounding the kinetics, dynamics, and isotope signature of ozone.

Communication: Enhanced oxygen reduction reaction and its underlying mechanism in PdIrCo trimetallic alloys
View Description Hide DescriptionBased on a combined density functional theory and experimental study, we present that the electrochemical activity of Pd3Co alloy catalysts toward oxygen reduction reaction (ORR) can be enhanced by adding a small amount of Ir. While Ir tends to favorably exist in the subsurface layers, the underlying Ir atoms are found to cause a substantial modification in the surface electronic structure. As a consequence, we find that the activation barriers of O/OH hydrogenation reactions are noticeably lowered, which would be mainly responsible for the enhanced ORR activity. Furthermore, our study suggests that the presence of Ir in the nearsurface region can suppress Co outdiffusion from the Pd3Co substrate, thereby improving the durability of PdIrCo catalysts. We also discuss the relative roles played by Ir and Co in enhancing the ORR activity relative to monometallic Pd catalysts.
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 ARTICLES

 Theoretical Methods and Algorithms

Comparison between Gaussiantype orbitals and plane wave ab initio density functional theory modeling of layer silicates: Talc [Mg_{3}Si_{4}O_{10}(OH)_{2}] as model system
View Description Hide DescriptionThe quantum chemical characterization of solid state systems is conducted with many different approaches, among which the adoption of periodic boundary conditions to deal with threedimensional infinite condensed systems. This method, coupled to the Density Functional Theory (DFT), has been proved successful in simulating a huge variety of solids. Only in relatively recent years this ab initio quantummechanic approach has been used for the investigation of layer silicate structures and minerals. In the present work, a systematic comparison of different DFT functionals (GGAPBEsol and hybrid B3LYP) and basis sets (plane waves and allelectron Gaussiantype orbitals) on the geometry, energy, and phonon properties of a model layer silicate, talc [Mg 3Si4O10(OH)2], is presented. Long range dispersion is taken into account by DFT+D method. Results are in agreement with experimental data reported in literature, with minimal deviation given by the GTO/B3LYPD* method regarding both axial lattice parameters and interaction energy and by PW/PBED for the unitcell volume and angular values. All the considered methods adequately describe the experimental talc infrared spectrum.

Multicomponent symmetryprojected approach for molecular ground state correlations
View Description Hide DescriptionThe symmetryprojected Hartree–Fock ansatz for the electronic structure problem can efficiently account for static correlation in molecules, yet it is often unable to describe dynamic correlation in a balanced manner. Here, we consider a multicomponent, systematically improvable approach, that accounts for all ground state correlations. Our approach is based on linear combinations of symmetryprojected configurations built out of a set of nonorthogonal, variationally optimized determinants. The resulting wavefunction preserves the symmetries of the original Hamiltonian even though it is written as a superposition of deformed (brokensymmetry) determinants. We show how short expansions of this kind can provide a very accurate description of the electronic structure of simple chemical systems such as the nitrogen and the water molecules, along the entire dissociation profile. In addition, we apply this multicomponent symmetryprojected approach to provide an accurate interconversion profile among the peroxo and bis(μoxo) forms of [Cu 2O2]^{2+}, comparable to other stateoftheart quantum chemical methods.

Permutation invariant polynomial neural network approach to fitting potential energy surfaces. II. Fouratom systems
View Description Hide DescriptionA rigorous, general, and simple method to fit global and permutation invariant potential energy surfaces (PESs) using neural networks (NNs) is discussed. This socalled permutation invariant polynomial neural network (PIPNN) method imposes permutation symmetry by using in its input a set of symmetry functions based on PIPs. For systems with more than three atoms, it is shown that the number of symmetry functions in the input vector needs to be larger than the number of internal coordinates in order to include both the primary and secondary invariant polynomials. This PIPNN method is successfully demonstrated in three atomtriatomic reactive systems, resulting in fulldimensional global PESs with average errors on the order of meV. These PESs are used in fulldimensional quantum dynamical calculations.

On the ground state calculation of a manybody system using a selfconsistent basis and quasiMonte Carlo: An application to water hexamer
View Description Hide DescriptionGiven a quantum manybody system, the SelfConsistent Phonons (SCP) method provides an optimal harmonic approximation by minimizing the free energy. In particular, the SCP estimate for the vibrational ground state (zero temperature) appears to be surprisingly accurate. We explore the possibility of going beyond the SCP approximation by considering the system Hamiltonian evaluated in the harmonic eigenbasis of the SCP Hamiltonian. It appears that the SCP ground state is already uncoupled to all singly and doublyexcited basis functions. So, in order to improve the SCP result at least triplyexcited states must be included, which then reduces the error in the ground state estimate substantially. For a multidimensional system two numerical challenges arise, namely, evaluation of the potential energy matrix elements in the harmonic basis, and handling and diagonalizing the resulting Hamiltonian matrix, whose size grows rapidly with the dimensionality of the system. Using the example of water hexamer we demonstrate that such calculation is feasible, i.e., constructing and diagonalizing the Hamiltonian matrix in a triplyexcited SCP basis, without any additional assumptions or approximations. Our results indicate particularly that the ground state energy differences between different isomers (e.g., cage and prism) of water hexamer are already quite accurate within the SCP approximation.

Charge asymmetry in rovibrationally excited HD^{+} determined using explicitly correlated allparticle Gaussian functions
View Description Hide DescriptionVery accurate nonBornOppenheimer quantummechanical calculations are performed to determine the average values of the interparticle distances and the protondeuteron density function for the rovibrationally excited HD^{+} ion. The states corresponding to excitations to all bound vibrational states (v = 0, …, 22) and simultaneously excited to the first excited rotational state (N = 1) are considered. To describe each state up to 8000 explicitly correlated allparticle Gaussian functions are used. The nonlinear parameters of the Gaussians are variationally optimized using a procedure that employs the analytical energy gradient determined with respect to these parameters. The results show an increasing asymmetry in the electron distribution with the vibrational excitation as the electron density shifts towards deuteron and away from the proton.

Flexible nuclear screening approximation to the twoelectron spin–orbit coupling based on ab initio parameterization
View Description Hide DescriptionThe derivation, implementation, and validation of a new approximation to the twoelectron spin–orbit coupling (SOC) terms is reported. The approximation, referred to as flexible nuclear screening spin–orbit, is based on the effective oneelectron spin–orbit operator and accounts for twoelectron SOC effects by screening nuclear charges. A highly flexible scheme for the nuclear screening is developed, mainly using parameterization based on ab initio atomic SOC calculations. Tabulated screening parameters are provided for contracted and primitive Gaussiantype basis functions of the ANORCC basis set for elements from H to Cm. The strategy for their adaptation to any other Gaussian basis set is presented and validated. A model to correct for the effect of splitting of transition metal d orbitals on their SOC matrix elements is introduced. The method is applied to a representative set of molecules, and compared to exact treatment and other approximative approaches at the same level of relativistic theory. The calculated SOC matrix elements are in very good agreement with their “exact” values; deviation below 1% is observed on average. The presented approximation is considered to be generally applicable, simple to implement, highly efficient, and accurate.

Automatic computer procedure for generating exact and analytical kinetic energy operators based on the polyspherical approach: General formulation and removal of singularities
View Description Hide DescriptionWe present new techniques for an automatic computation of the kinetic energy operator in analytical form. These techniques are based on the use of the polyspherical approach and are extended to take into account Cartesian coordinates as well. An automatic procedure is developed where analytical expressions are obtained by symbolic calculations. This procedure is a full generalization of the one presented in Ndong et al. , [J. Chem. Phys.136, 034107 (2012)]. The correctness of the new implementation is analyzed by comparison with results obtained from the TNUM program. We give several illustrations that could be useful for users of the code. In particular, we discuss some cyclic compounds which are important in photochemistry. Among others, we show that choosing a welladapted parameterization and decomposition into subsystems can allow one to avoid singularities in the kinetic energy operator. We also discuss a relation between polyspherical and Zmatrix coordinates: this comparison could be helpful for building an interface between the new code and a quantum chemistry package.

Origin of parameter degeneracy and molecular shape relationships in geometricflow calculations of solvation free energies
View Description Hide DescriptionImplicit solvent models are important tools for calculating solvation free energies for chemical and biophysical studies since they require fewer computational resources but can achieve accuracy comparable to that of explicitsolvent models. In past papers, geometric flowbased solvation models have been established for solvation analysis of small and large compounds. In the present work, the use of realistic experimentbased parameter choices for the geometric flow models is studied. We find that the experimental parameters of solvent internal pressure p = 172 MPa and surface tension γ = 72 mN/m produce solvation free energies within 1 RT of the global minimum rootmeansquared deviation from experimental data over the expanded set. Our results demonstrate that experimental values can be used for geometric flow solvent model parameters, thus eliminating the need for additional parameterization. We also examine the correlations between optimal values of p and γ which are strongly anticorrelated. Geometric analysis of the small molecule test set shows that these results are interconnected with an approximately linear relationship between area and volume in the range of molecular sizes spanned by the data set. In spite of this considerable degeneracy between the surface tension and pressure terms in the model, both terms are important for the broader applicability of the model.

Stochastic thermodynamics of fluctuating density fields: Nonequilibrium free energy differences under coarsegraining
View Description Hide DescriptionWe discuss the stochastic thermodynamics of systems that are described by a timedependent density field, for example, simple liquids and colloidal suspensions. For a timedependent change of external parameters, we show that the Jarzynski relation connecting work with the change of free energy holds if the time evolution of the density follows the KawasakiDean equation. Specifically, we study the work distributions for the compression and expansion of a twodimensional colloidal model suspension implementing a practical coarsegraining scheme of the microscopic particle positions. We demonstrate that even if coarsegrained dynamics and density functional do not match, the fluctuation relations for the work still hold albeit for a different, apparent, change of free energy.

Orbitaloptimized density cumulant functional theory
View Description Hide DescriptionIn density cumulant functional theory (DCFT) the electronic energy is evaluated from the oneparticle density matrix and twoparticle density cumulant, circumventing the computation of the wavefunction. To achieve this, the oneparticle density matrix is decomposed exactly into the meanfield (idempotent) and correlation components. While the latter can be entirely derived from the density cumulant, the former must be obtained by choosing a specific set of orbitals. In the original DCFT formulation [W. Kutzelnigg, J. Chem. Phys.125, 171101 (2006)] the orbitals were determined by diagonalizing the effective Fock operator, which introduces partial orbital relaxation. Here we present a new orbitaloptimized formulation of DCFT where the energy is variationally minimized with respect to orbital rotations. This introduces important energy contributions and significantly improves the description of the dynamic correlation. In addition, it greatly simplifies the computation of analytic gradients, for which expressions are also presented. We offer a perturbative analysis of the new orbital stationarity conditions and benchmark their performance for a variety of chemical systems.
 Atoms, Molecules, and Clusters

Hydrogen evolution from water through metal sulfide reactions
View Description Hide DescriptionTransition metal sulfides play an important catalytic role in many chemical reactions. In this work, we have conducted a careful computational study of the structures, electronic states, and reactivity of metal sulfide cluster anions M2SX ^{ − } (M = Mo and W, X = 4–6) using density functional theory. Detailed structural analysis shows that these metal sulfide anions have ground state isomers with two bridging sulfide bonds, notably different in some cases from the corresponding oxides with the same stoichiometry. The chemical reactivity of these metal sulfide anions with water has also been carried out. After a thorough search on the reactive potential energy surface, we propose several competitive, energetically favorable, reaction pathways that lead to the evolution of hydrogen. Selectivity in the initial water addition and subsequent hydrogen migration are found to be the key steps in all the proposed reaction channels. Initial adsorption of water is most favored involving a terminal metal sulfur bond in Mo 2S4 ^{−} isomers whereas the most preferred orientation for water addition involves a bridging metal sulfur bond in the case of W2S4 ^{−} and M2S5 ^{−} isomers. In all the lowest energy H2 elimination steps, the interacting hydrogen atoms involve a metal hydride and a metal hydroxide (or thiol) group. We have also observed a higher energy reaction channel where the interacting hydrogen atoms in the H2 elimination step involve a thiol (–SH) and a hydroxyl (–OH) group. For all the reaction pathways, the Mo sulfide reactions involve a higher barrier than the corresponding W analogues. We observe for both metals that reactions of M2S4 ^{−} and M2S5 ^{−} clusters with water to liberate H2 are exothermic and involve modest free energy barriers. However, the reaction of water with M2S6 ^{−} is highly endothermic with a considerable barrier due to saturation of the local bonding environment.

Matrix isolation infrared spectroscopic and theoretical study of 1,1,1trifluoro2chloroethane (HCFC133a)
View Description Hide DescriptionThe molecular structure and infrared spectrum of the atmospheric pollutant 1,1,1trifluoro2chloroethane (HCFC133a; CF3CH2Cl) in the ground electronic state were characterized experimentally and theoretically. Excited state calculations (at the CASSCF, MRCISD, and MRCISD+Q levels) have also been performed in the range up to ∼9.8 eV. The theoretical calculations show the existence of one (staggered) conformer, which has been identified spectroscopically for the monomeric compound isolated in cryogenic (∼10 K) argon and xenon matrices. The observed infrared spectra of the matrixisolated HCFC133a were interpreted with the aid of MP2/augccpVTZ calculations and normal coordinate analysis, which allowed a detailed assignment of the observed spectra to be carried out, including identification of bands due to different isotopologues (^{35}Cl and ^{37}Cl containing molecules). The calculated energies of the several excited states along with the values of oscillator strengths and previous results obtained for CFCs and HCFCs suggest that the previously reported photolyses of the title compound at 147 and 123.6 nm [T. Ichimura, A. W. Kirk, and E. TschuikowRoux, J. Phys. Chem.81, 1153 (1977)] are likely to be initiated in the n4s and n4p Rydberg states, respectively.

Environmental effects on noblegas hydrides: HXeBr, HXeCCH, and HXeH in noblegas and molecular matrices
View Description Hide DescriptionNoblegas hydrides HNgY (Ng is a noblegas atom and Y is an electronegative group) are sensitive probes of local environment due to their relatively weak bonding and large dipole moments. We experimentally studied HXeBr in Ar, Kr, and N2 matrices, HXeCCH in Ne and N2 matrices, and HXeH in an N2 matrix. These are the first observations of noblegas hydrides in an N2 matrix. An N2 matrix strongly increases the H–Xe stretching frequency of HXeBr and HXeCCH with respect to a Ne matrix, which is presumably due to a strong interaction between the HNgY dipole moment and quadrupole moments of the surrounding lattice N2 molecules. The spectral shift of HXeBr in an N2 matrix is similar to that in a CO2 matrix, which is a rather unexpected result because the quadrupole moment of CO2 is about three times as large as that of N2. The H–Xe stretching frequencies of HXeBr and HXeCCH in noblegas matrices show a trend of ν(Ne) < ν(Xe) < ν(Kr) < ν(Ar), which is a nonmonotonous function of the dielectric constants of the noblegas solids. The MP2(full) calculations of HXeBr and HXeCCH with the polarizable continuum model as well as the CCSD(T) calculations of the HXeBr···Ng and HXeCCH···Ng (Ng = Ne, Ar, Kr, and Xe) complexes cannot fully explain the experimental observations. It is concluded that more sophisticated computational models should be used to describe these experimental findings.

Rotational relaxation of CS by collision with ortho and paraH_{2} molecules
View Description Hide DescriptionQuantum mechanical investigation of the rotationally inelastic collisions of CS with ortho and paraH2 molecules is reported. The new global fourdimensional potential energy surface presented in our recent work is used. Close coupling scattering calculations are performed in the rigid rotor approximation for ortho and paraH2 colliding with CS in the j = 0–15 rotational levels and for collision energies ranging from 10^{−2} to 10^{3} cm^{−1}. The cross sections and rate coefficients for selected rotational transitions of CS are compared with the ones previously reported for the collision of CS with He. The largest discrepancies are observed at low collision energy, below 1 cm^{−1}. Above 10 cm^{−1}, the approximation using the square root of the relative mass of the colliders to calculate the cross sections between a molecule and H2 from the data available with ^{4}He is found to be a good qualitative approximation. The rate coefficients calculated with the electron gas model for the HeCS system show more discrepancy with our accurate results. However, scaling up these rates by a factor of 2 gives a qualitative agreement.

A global ab initio potential energy surface for the X ^{ 2} A ^{′} ground state of the Si + OH → SiO + H reaction
View Description Hide DescriptionWe report the first global potential energy surface (PES) for the X ^{ 2} A ^{′} ground electronic state of the Si(^{3}P) + OH(X^{2}Π) → SiO( ) + H(^{2}S) reaction. The PES is based on a large number of ab initio energies obtained from multireference configuration interaction calculations plus Davidson correction (MRCI+Q) using basis sets of quadruple zeta quality. Corrections were applied to the ab initio energies in the reactant channel allowing a proper description of longrange interactions between Si(^{3}P) and OH(X^{2}Π). An analytical representation of the global PES has been developed by means of the reproducing kernel Hilbert space method. The reaction is found barrierless. Two minima, corresponding to the SiOH and HSiO isomers, and six saddle points, among which the isomerization transition state, have been characterized on the PES. The vibrational spectra of the SiOH/HSiO radicals have been computed from secondorder perturbation theory and quantum dynamics methods. The structural, energetic, and spectroscopic properties of the two isomers are in good agreement with experimental data and previous high quality calculations.

Correlation between oxygen adsorption energy and electronic structure of transition metal macrocyclic complexes
View Description Hide DescriptionOxygen adsorption energy is directly relevant to the catalytic activity of electrocatalysts for oxygen reduction reaction (ORR). In this study, we established the correlation between the O2 adsorption energy and the electronic structure of transition metal macrocyclic complexes which exhibit activity for ORR. To this end, we have predicted the molecular and electronic structures of a series of transition metal macrocyclic complexes with planar N4 chelation, as well as the molecular and electronic structures for the O2 adsorption on these macrocyclic molecules, using the density functional theory calculation method. We found that the calculated adsorption energy of O2 on the transition metal macrocyclic complexes was linearly related to the average position (relative to the lowest unoccupied molecular orbital of the macrocyclic complexes) of the nonbonding d orbitals ( , and d yz ) which belong to the central transition metal atom. Importantly, our results suggest that varying the energy level of the nonbonding d orbitals through changing the central transition metal atom and/or peripheral ligand groups could be an effective way to tuning their O2 adsorption energy for enhancing the ORR activity of transition metal macrocyclic complex catalysts.