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We have calculated the anharmonic OH-(aq) vibrational spectrum in aqueous solution with a 'classical Monte Carlo simulation + QM/MM + vibrational' sequential approach. A new interaction model was used in the Monte Carlo simulations: a modified version of the charged-ring hydroxide-water model from the literature. This spectrum is compared with experiment and with a spectrum based on CPMD-generated structures, and the hydration structures and H-bonding for the two models are compared. We find that: (i) the solvent-induced frequency shift as well as the absolute OH- frequency are in good agreement with experiment using the two models; (ii) the Raman and IR bands are very similar, in agreement with experiment; (iii) the hydration structure and H-bonding around the ion are very different with the two ion-water interaction models (charged-ring and CPMD); (iv) a cancellation effect between different regions of the hydration shell makes the total spectra similar for the two interaction models, although their hydration structures are different; (v) the net OH- frequency shift is a blueshift of about +80 cm-1 with respect to frequency of the gas-phase ion. [ABSTRACT FROM AUTHOR]

Results from first-principles calculations present a rather clear picture of the interaction of SO2 with unreduced and partially reduced (111) and (110) surfaces of ceria. The Ce3+/Ce4+ redox couple, together with many oxidation states of S, give rise to a multitude of SOx species, with oxidation states from +III to +VI. SO2 adsorbs either as a molecule or attaches via its S-atom to one or two surface oxygens to form sulfite (SO32-) and sulfate (SO42-) species, forming new S-O bonds but never any S-Ce bonds. Molecular adsorption is found on the (111) surface. SO32- structures are found on both the (111) and (110) surfaces of both stoichiometric and partially reduced ceria. SO42- structures are observed on the (110) surface together with the formation of two reduced Ce3+ surface cations. SO2 can also partially heal the ceria oxygen vacancies by weakening a S-O bond, when significant electron transfer from the surface (Ce4f) into the lowest unoccupied molecular orbital of the SO2 adsorbate takes place and oxidizes the surface Ce3+ cations. Furthermore, we propose a mechanism that could lead to monodentate sulfate formation at the (111) surface. [ABSTRACT FROM AUTHOR]

The anharmonic OH stretching vibrational frequencies, ν(OH), for the first-shell water molecules around the Li+, Ca2+, Mg2+, and Al3+ ions in dilute aqueous solutions have been calculated based on classical molecular dynamics (MD) simulations and quantum-mechanical (QM) calculations. For Li+(aq), Ca2+(aq), Mg2+(aq), and Al3+(aq), our calculated IR frequency shifts, Δν(OH), with respect to the gas-phase water frequency, are about -300, -350, -450, and -750 cm-1, compared to -290, -290, -420, and -830 cm-1 from experimental infrared (IR) studies. The agreement is thus quite good, except for the order between Li+ and Ca2+. Given that the polarizing field from the Ca2+ ion ought to be larger than that from Li+(aq), our calculated result seems reasonable. Also the absolute OH frequencies agree well with experiment. The method we used is a sequential four-step procedure: QM(electronic) to make a force field+MD simulation+QM(electronic) for point-charge-embedded Mn+ (H2O)ysecond shell (H2O)zthird shell clusters+QM(vibrational) to yield the OH spectrum. The many-body Ca2+-water force-field presented in this paper is new. IR intensity-weighting of the density-of-states frequency distributions was carried out by means of the squared dipole moment derivatives. [ABSTRACT FROM AUTHOR]

Anharmonic OH phonon dispersion curves have been calculated for the Mg(OH)2 crystal. A crystal Hamiltonian was set up for the vibrational problem, where the coordinates consists of the bond lengths of two hydroxide ions in the central unit cell. Its two-dimensional potential energy surface was constructed from first principle calculations within the density functional theory approximation. Dispersion curves were calculated by diagonalizing the Hamiltonian in a basis of singly excited crystal functions. The single particle functions used to construct the crystal states were taken from a Morse oscillator basis set. These well chosen functions made it possible to restrict calculations to include only very few functions, which greatly contributed to a transparent presentation of the underlying theory. All calculations could be done analytically except for the calculation of a few integrals. We have compared our results with those of a series of harmonic lattice dynamics calculations and have found that the anharmonicity shifts the IR and Raman dispersion curves downward appreciably and slightly changes the energy differences between both curves. From an analysis of the harmonic results we conclude that incorporating the coupling between OH stretching motion and the motion of their centers of mass will appreciably change the overall features of the dispersion curves. Extension of the anharmonic model along these lines will cause no problem to the theoretical approach presented in this paper. [ABSTRACT FROM AUTHOR]

In this paper we evaluate the performance of density functional theory with the B3LYP functional for calculations on ceria (CeO2) and cerium sesquioxide (Ce2O3). We demonstrate that B3LYP is able to describe CeO2 and Ce2O3 reasonably well. When compared to other functionals, B3LYP performs slightly better than the hybrid functional PBE0 for the electronic properties but slightly worse for the structural properties, although neither performs as well as LDA+U(U=6 eV) or PBE+U(U=5 eV). We also make an extensive comparison of atomic basis sets suitable for periodic calculations of these cerium oxides. Here we conclude that there is currently only one type of cerium basis set available in the literature that is able to give a reasonable description of the electronic structure of both CeO2 and Ce2O3. These basis sets are based on a 28 electron effective core potential (ECP) and 30 electrons are attributed to the valence space of cerium. Basis sets based on 46 electron ECPs fail for these materials. [ABSTRACT FROM AUTHOR]

A two-dimensional quantum-mechanical vibrational model has been used to calculate the anharmonic OH vibrational frequencies in the layered Mg(OH)2 (brucite) crystal. The underlying potential energy surface was generated by density functional theory (DFT) calculations. The resulting OH frequencies are upshifted (blueshifted) by about +75 cm-1 with respect to the gas-phase OH frequency (+120 cm-1 in experiments; the discrepancy is mainly due to inadequacies in the DFT and pseudopotential models). The Raman-IR split is about 50 cm-1, both in the calculations and in experiments. We find that the blueshift phenomenon in brucite can qualitatively be explained by a parabolalike “OH frequency versus electric field” correlation curve pertaining to an OH- ion exposed to an electric field. We also find that it is primarily the neighbors within the Mg(OH)2 layer that induce the blueshift while the interlayer interaction gives a smaller (and redshifting) contribution. [ABSTRACT FROM AUTHOR]

Anharmonic vibrational frequencies for the Raman-active (A1g) and the IR-active (A2u) modes have been calculated for the LiOH crystal within a plane-wave density functional theory (DFT) framework. We find that a two-dimensional quantum-mechanical vibrational approach, allowing for anharmonic coupling between symmetric and antisymmetric OH stretching modes, produces OH frequencies—both absolute frequencies and gas-to-solid frequency shifts—in good agreement with experiment. Remaining errors in the absolute frequencies are largely a consequence of the DFT model chosen. A one-dimensional normal-mode following vibrational treatment, on the other hand, fails to reproduce both absolute anharmonic frequencies and gas-to-solid frequency shifts. [ABSTRACT FROM AUTHOR]

We have combined an embedded-cluster model with an extension of the method of increments to treat the adsorption of molecules on a surface. In this way we are able to investigate the physisorption of CO on CeO2(110) at the MP2, MP4(SDTQ), and CCSD(T) levels with only moderate computational costs. We find that, at the CCSD(T) level, 25% of the adsorption energy originates from electron correlation. The interactions of the CO molecule with its five nearest cerium and oxygen neighbors in the surface layer make the largest contributions to the electron correlation. Approximately 97% of the adsorption-induced electron correlation energy part of the adsorption energy is recovered by the method of increments (in our chosen expansion), at the MP2 level. [ABSTRACT FROM AUTHOR]

Using the DFT+U method, i.e., first principles density functional theory calculations with the inclusion of on-site Coulomb interaction, the effects of Pd doping on the O vacancy formation energy (Evac) in CeO2 has been studied. We find that Evac is lowered from 3.0 eV in undoped ceria to 0.6 eV in the Pd-doped compound. Much of this decrease can be attributed to emerging Pd-induced gap states above the valence band and below the empty Ce 4f states. These localized defect states involve the Pd ion and its nearest neighbors, which are also the main acceptors of the extra electrons left on reduction. The effect of the Pd dopant on the geometric structure is very modest for CeO2 but considerable for CeO2-x. [ABSTRACT FROM AUTHOR]

Ab initio embedded-cluster calculations have been performed for the CeO2(110) surface using temperature induced structures from molecular dynamics (MD) snapshots. As a first step towards understanding how temperature induced distortions of the surface structure influence the surface oxygen reactivity, the energy cost of removing an O atom from the surface was calculated for 41 snapshots from the MD simulation at 300 K. The quantum mechanical embedded-cluster calculations show that already at 300 K the dynamics causes significant fluctuations (root mean square of 0.37 eV) in the O vacancy formation energy (Evac) while the distribution of the two excess electrons associated with the vacancy is virtually unaffected by the surface dynamics and remains localized on the two Ce ions close to the vacancy. It is also found that the quantum mechanical Evac fluctuations can be reproduced by oxygen vacancy calculations using only the relaxed shell-model force field (FF) itself and the MD geometries. Using the FF as the interaction model, the effect of raising the temperature to 750 K and the effect of doping with Ca were investigated for the oxygen vacancy formation. [ABSTRACT FROM AUTHOR]

The Zr doping in CeO2 may change the reduction properties and therefore the redox properties of CeO2. Using first-principles density functional theory with the inclusion of on-site Coulomb interaction for a 96-atom supercell, these effects are studied by comparing the differences in atomic structures, electronic structures, and reduction energies of the doped CeO2 and those of the nondoped CeO2. It is found that (1) Zr doping of the ceria structure results in important modifications involving nonequivalent O atoms; (2) the oxygen anions (still four-coordinated) next to the doping center show considerably lower reduction energies (by 0.6 eV) and larger displacements (“higher mobilities”); (3) an O vacancy is most easily created close to the Zr centers, therefore the Zr-doping centers might serve as nucleation centers for vacancy clustering; and (4) the electrons left by the released oxygen localize on two Ce cations neighboring the vacancy, which results in the reduction of two Ce4+ ions. [ABSTRACT FROM AUTHOR]

The optimized geometry, energetics, and vibrational properties of Al(D2O)n3+ clusters, with n=1,2,4, and 6, have been studied using plane waves, different local basis sets, different methodologies [density-functional theory, MP2, CCSD(T)], and different functionals (BLYP, PBE). Moreover, Car-Parrinello molecular-dynamics (MD) simulations using the BLYP functional, plane waves, and the Vanderbilt ultrasoft pseudopotentials have been performed for an aqueous Al3+ solution with 1 ion and 32 D2O molecules in a periodic box at room temperature, studied for 10 ps. The cluster calculations were performed to pinpoint possible shortcomings of the electronic structure description used in the Car-Parinello MD (CPMD) simulation. For the clusters, the hydration structure and interaction energies calculated with the ‘BLYP/plane-wave’ approach agree well with high-level ab initio methods but the exchange-correlation functional introduces errors in the OD stretching frequencies (both in the absolute values and in the ion-induced shifts). For the aqueous solution, the CPMD simulation yields structural properties in good agreement with experimental data. The CPMD-simulated OD stretching vibrational band for the first-shell water molecules around Al3+ is strongly downshifted by the influence of the ion and is compared with experimental data from the literature. To make such a comparison meaningful, the influences of a number of systematic effects have been addressed, such as the exchange-correlation functional, the fictitious electron mass, anharmonicity effects, and the small box size in the simulation. Each of these factors (except the last one) is found to affect the OD frequency by 100 cm-1 or more. The final “corrected” frequencies agree with experiment within ∼30 cm-1 for bulk water but are too little downshifted for the first-shell Al3+(aq) water molecules (by ∼200 cm-1). [ABSTRACT FROM AUTHOR]

The atomic and electronic structure of (111), (110), and (100) surfaces of ceria (CeO2) were studied using density-functional theory within the generalized gradient approximation. Both stoichiometric surfaces and surfaces with oxygen vacancies (unreduced and reduced surfaces, respectively) have been examined. It is found that the (111) surface is the most stable among the considered surfaces, followed by (110) and (100) surfaces, in agreement with experimental observations and previous theoretical results. Different features of relaxation are found for the three surfaces. While the (111) surface undergoes very small relaxation, considerably larger relaxations are found for the (110) and (100) surfaces. The formation of an oxygen vacancy is closely related to the surface structure and occurs more easily for the (110) surface than for (111). The preferred vacancy location is in the surface layer for CeO2(110) and in the subsurface layer (the second O-atomic layer) for CeO2(111). For both surfaces, the O vacancy forms more readily than in the bulk. An interesting oscillatory behavior is found for the vacancy formation energy in the upper three layers of CeO2(111). Analysis of the reduced surfaces suggests that the additional charge resulting from the formation of the oxygen vacancies is localized in the first three layers of the surface. Furthermore, they are not only trapped in the 4f states of cerium. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Many-body potentials for the aqueous Li[sup +], Na[sup +], Mg[sup 2+], and Al[sup 3+] ions have been constructed from ab initio cluster calculations. Pure pair, effective pair, effective three-body, and effective polarizable models were created and used in subsequent molecular dynamics simulations. The structures of the first and second solvation shells were studied using radial distribution functions and angular-radial distribution functions. The effective three-body and polarizable potentials yield similar first-shell structures, while the contraction of the O–O distances between the first and second solvation shells is more pronounced with the polarizable potentials. The definition of the tilt angle of the water molecules around the ions is discussed. When a proper definition is used, it is found that for Li[sup +], Mg[sup 2+], and Al[sup 3+] the water molecules prefer a trigonal orientation, but for Na[sup +] a tetrahedral orientation (ion in lone-pair direction) is preferred. The self-diffusion coefficients for the water molecules and the ions were calculated; the ionic values follow the order obtained from experiment, although the simulated absolute values are smaller than experiment for Mg[sup 2+] and Al[sup 3+]. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A method for the extraction of effective three-body potential parameters from high-level ab initio cluster calculations is presented and compared to effective pair potentials extracted at the same level. Dilute Li[sup +](aq) and Mg[sup 2+](aq) solutions are used as test cases and long molecular-dynamics simulations using these newly developed potentials were performed. Resulting thermodynamical, structural, and dynamical properties are compared to experiment as well as to the empirical effective pair potentials of Åqvist. Moreover, a new time-saving method for the correction of cluster energies computed with a relatively cheap ab initio method, to yield expensive, high-level ab initio energies, is presented. The effective pair approach is shown to give inconsistent results when compared to the effective three-body potentials. The performance of three different charge compensation methods (uniform charge plasm, Bogusz net charge correction, and counter ions) is compared for a large number of different system sizes. For most properties studied here, the system-size dependence is found to be small for system sizes with 256 water molecules or more. However, for the self-diffusion coefficients, a 1/L dependence is found, i.e., a very large system-size dependence. A very simple method for correcting for this deficiency is proposed. The results for most properties are found to compare reasonably well to experiment when using the effective three-body potentials. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Potential energy hypersurfaces (PESs) for four fluoroform complexes (with acetonitrile, ethyleneoxide, formaldehyde, and water) were explored at the HF, MP2, and B3LYP/6-311 ++ G(d,p) levels of theory. Anharmonic C-H stretching vibrational frequency shifts are reported for all minima located on the studied PESs. In all cases, the lowest-energy minimum occurs for a C-H…O(N) hydrogen-bonded arrangement and is characterized by a significant C-H frequency blueshift (upshift), while additional minima [for "reversed" orientations, in which there is no direct C-H…O(N) contact] show only small C-H frequency upshifts. The large blueshifts found for the hydrogen-bonded arrangements are predominantly caused by the electronic exchange interaction, as revealed by Kitaura-Morokuma (KM) analysis, while the purely electrostatic1polarization interaction leads to C-H frequency redshifts, which was proven both by the KM analysis and the charge field perturbational (CFP) approach. The large net blueshifting effect of the exchange contribution is only possible thanks to the smallness of the redshifting electrostatic1polarization contribution, which, in turn, is a consequence of the fact that dμ[SUP(0)]/dr[SUBCH] is negative for the fluoroform molecule. In all cases the charge transfer as well as the dispersion energy contributions lead to frequency redshifts. The small C-H blueshifts for the reversed orientations are almost completely governed by the electrostatic interaction, as shown by the KM and CFP analyses. All these characteristics of blueshifting H bonds are in line with a previously outlined general model concerning "standard" and "blueshifting" H-bonding interactions [K. Hermansson, J. Phys. Chem. A 106, 4695 (2002)]. [ABSTRACT FROM AUTHOR]

The classical molecular dynamics (MD) and quantum mechanics (QM) embedded-cluster calculations are combined to investigate the adsorption of CO on a dynamically distorted MgO(001) surface. The distribution of adsorption energies for CO above the Mg(super 2+) sites on the MgO(001) surface at 50, 80, and 150 K is predicted by using the linear expression.

A Car-Parrinello simulation of a single Fe(super 3+) ion in periodic box of 32 water molecules is presented. The aim is to investigate the hydration shell structure and water vibrations for the hydrated ferric ion.

The average Oh stretching vibrational frequency for the water molecules in the first hydration shell around a Li(super +) ion in a dilute aqueous solution is calculated by a hybrid molecular dynamics + quantum-mechanical ('MD+QM') approach. The average frequency at the B3LYP level of theory, including second- and third-shell effects as well as anharmonicity, falls at approximately 3380cm (super -1), and the calculated downshift is approximately -305 cm(super -1).

The structure and dynamics of water molecules around the Fe(super 2+) and Fe(super 3+) ions, as well as the ionic diffusion is discussed, using a new flexible water model (SPC+CCL) combined with ion-water potential. It is concluded that the flexible-water model SPC+CCL describes the structural properties and the diffusion equally well or better than the SPC/E water model for the Fe(super 2+)(ag) and Fe(super 3+)(ag) solutions.

Studies molecular dipole moments, charges and difference electron densities from Hartree-Fock calculations for individual molecule along geometry-optimized water chains. Complementarity of the properties; Convergence of the polarization for the molecules at the ends of the chains compared to the middle.

Examines the ab initio calculations for H[sub 2] adsorption on temperature-reconstructed MgO(111) surfaces based from a molecular dynamics (MD) simulation. Physi- and chemisorption properties for the H[sub 2]/MgO(111) system; Importance of the chemisorption of H[sub 2] in oxide surfaces; Account on MD simulations.