Ab Initio Cluster Research Articles

Ordering and phase separation around Ti 50Al 25Mo 25 composition in ternary Ti–Al–Mo bcc alloys

The phase equilibria of bcc-based phases in the Ti–Al–Mo alloy system has been studied from first-principles using a combination of ab initio total energy and cluster variation method (CVM) calculations. A set of effective cluster interaction parameters has been derived from the total energies, already computed, of 18 binary and ternary bcc superstructures. These interaction parameters were the input for CVM computation of alloy thermodynamics properties. The CVM has been used to determine the bcc composition–temperature phase diagram in the Ti–Al–Mo system and site preference for bcc-based phases. The investigation focuses its attention on the discussion about the formation of a two-phase field A2+B2 around the Ti 50Al 25Mo 25 composition to suggest some potential directions for future research and development activities on high temperature bcc based superalloys.

Charge transport in micas: The kinetics of FeII/III electron transfer in the octahedral sheet

The two principal FeII/III electron exchange reactions underlying charge transport in the octahedral sheet of ideal end-member annite were modeled using a combination of ab initio calculations and Marcus electron transfer theory. A small polaron model was applied which yielded electron hopping activation energies that agree well with the limited available experimental data. A small ab initio cluster model successfully reproduced several important structural, energetic, and magnetic characteristics of the M1 and M2 Fe sites in the annite octahedral sheet. The cluster enabled calculation of the internal reorganization energy and electronic coupling matrix elements for the M2–M2 and M1–M2 electron transfer reactions. The M2–M2 electron transfer is symmetric with a predicted forward/reverse electron hopping rate of 106 s−1. The M1–M2 electron transfers are asymmetric due to the higher ionization potential by 0.46 eV of FeII in the M1 site. The electronic coupling matrix elements for these reactions are predicted to be small and of similar magnitude, suggesting the possibility that the coupling is essentially direction independent amongst hopping directions in the octahedral sheet. M1 Fe sites are predicted to be efficient electron traps and charge transport should occur by nearest-neighbor electron hops along the M2 Fe sublattice.

Effective three-body potentials for Li+(aq) and Mg2+(aq)

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+(aq) and Mg2+(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.

NUCLEAR MAGNETIC RESONANCE STUDY OF Al:Si AND F:OH ORDER IN ZUNYITE

Zunyite [Al13Si5O20(OH,F)18Cl] possesses an unusual combination of an Al 13O16(OH)24 Keggin structure and a Si 5O16 pentamer unit. We used 19 F, 27 Al, 29 Si MAS and 27 Al MQMAS NMR spectroscopy of a series of samples with varying Al:Si and F:OH ratios to investigate the degree of atomic order in zunyite. The quadrupole coupling constant (C Q) and asymmetry parameter () obtained from simulations of 27 Al MAS and MQMAS spectra were compared with those derived from the electric field gradients (EFG), which were calculated from an ab initio cluster quantum model using the program Gaussian 98W. This comparison led to unequivocal assignments for 29 Si, 27 Al and 19 F MAS NMR peaks and showed that the number of Al substituting for Si(1) in the pentamer increases with Al content. The number of Al octahedra with one F atom increases with F content, whereas the number with two F atoms remains negligible, indicating that F preferentially occupies AlO 4OHF octahedra.

Six-dimensional ab initio potential energy surfaces for H3O+ and NH3: Approaching the subwave number accuracy for the inversion splittings

New potential energy surfaces are calculated for the hydronium ion using high-order coupled cluster ab initio methods. Large basis sets are used especially for the inversion part of the full surface. Electronic energies obtained with different correlation consistent basis sets are extrapolated to the infinite basis set limit. Core-valence and first order relativistic effects are also included. The influence of these two contributions and basis set sizes on both the inversion barrier height and equilibrium geometry are investigated thoroughly. The same methods are also adopted for ammonia in order to further improve a recently published surface [J. Chem. Phys. 118, 6358 (2003)]. The vibrational eigenvalues are calculated variationally both for the symmetric and asymmetric isotopomers using exact six-dimensional kinetic energy operators and successive basis set contractions. With the new surfaces, the mean absolute deviations obtained for all experimentally observed inversion splittings for different isotopomers of H3O+ (8 states) and NH314 (17 states) are 0.78 and 0.25 cm−1, respectively. Inversion levels are calculated with almost similar accuracy. These results indicate that the calculated inversion barrier heights for H3O+ and NH3, 650 and 1792 cm−1, respectively, are close to the real values. The value for ammonia is also close to the height determined from published experimental data in our previous work. The lowest energies for the high-frequency modes are computed with the mean absolute deviation being less than 2 cm−1 for isotopomers of H3O+ and less than 4.5 cm−1 for NH314 with respect to experimental energies.

Structural relationship between V 2O 5 (0 0 1) surface and the bulk: cluster bulk termination models

Ab initio periodic and cluster Hartree–Fock calculations have been performed to investigate the structural and electronic properties of both V 2O 5 bulk and ( 0 0 1 ) surface. A full bulk geometry optimization yields to good VO lengths which are found to be in agreement with experiment. The relationship between the bulk and surface structure is investigated by different cluster models with optimized bulk termination. The validity of this approach is discussed through the experimental and theoretical surface results.

Interactions of water and methanol with a mixture of copper and zinc metals: a theoretical ab initio study

Ab initio cluster quantum chemical calculations at the Hartree–Fock and second-order Moller–Plesset perturbation theory levels were carried out to mimic the interactions of water and methanol with a mixture of Cu and Zn metals. It was shown that both molecular and dissociative adsorption of methanol on a mixture of Cu and Zn metal catalyst are preferred over the corresponding adsorptions of water. Estimated transition-state structures for dissociation of methanol into CH·3 and OH· lie about 9.0 and 22.0 kcal/mol higher compared to the dissociated (forward reaction) and molecular adsorption (reverse reaction) complexes, respectively. Based on distinct radicals' bond energies with the active sites of the catalyst considered, it is suggested that hydrogen molecules could be formed through a chain of homogeneous reactions of methyl radicals released into the gas phase with the water and/or methanol molecules.

FTIR spectroscopic and ab initio evidence for an amphipathic character of CO bonding with silanol groups

FTIR spectroscopy provides evidence that carbon monoxide interacts with surface silanol groups of silica and silicalite to form both SiOH⋯CO and SiOH⋯OC hydrogen-bonded complexes. The C-bonded adduct shows a characteristic IR absorption band which is blue-shifted as compared to free CO (2143 cm −1), while the O-bonded adduct is characterized by a red-shifted band. Variable temperature IR spectroscopy has shown that these two hydrogen-bonded adducts are in a temperature-dependent equilibrium which involves an enthalpy change of about 2–3 kJ mol −1. Spectroscopic data are supported by ab initio cluster model calculations of different complexity levels.

Boron Segregation on the ${\rm Si}(111)\sqrt{3} \times \sqrt{3}{\rm R}30^\circ$ Surface

In this paper we report the results of calculations of the energies associated with the segregation of boron on the [Formula: see text] surface. These calculations have been carried out using the plane wave pseudopotential density functional code fhi98md in a periodic slab formalism. The segregation energy is predicted to be -0.77 eV. This prediction is intermediate between the "experimentally determined" values of -0.33 eV and -0.48 eV, and the values of -1.83 eV and -2.10 eV determined from AM1 cluster calculations. Additional information has been obtained by performing ab initio density functional cluster calculations using the Gaussian98 code. These latter results indicate that the AM1 calculations significantly overestimate the segregation energy of boron on the [Formula: see text] surface. They also provide strong support for the fhi98md calculations.

Ab initio cluster calculations of silica surface sites

Silica surface sites, which can be formed in cleavage processes, and their hydrolyzed counterparts are investigated with ab initio cluster calculations. Natural Bond Orbital (NBO) theory is used to characterize bonding around silica surface sites. Higher energy lone pairs of electrons on oxygen atoms either hyperconjugate to vicinal silanol/siloxane antibonding orbitals or backdonate electron density via donor–acceptor π-type bonding with participation of pd or p hybrids on silicon atoms. Upon substitution of hydroxyl groups of orthosilicic acid with silica monomers the strength of siloxane and silanol Si–O bonding increases as energies of bonding orbitals and contributions from p-orbitals decrease. Silanone sites and a complementary pair of silyl/siloxy radical sites are found to be the most stable geminal and single non-hydrolyzed sites, respectively. Atomic charges based on natural wavefunctions and on fitting to electrostatic potential, and characteristic bands of IR spectra associated with siloxane and silanol stretching vibrations of silica surface sites are reported.

Characterization of ArnO− clusters from ab initio and diffusion Monte Carlo calculations

The structure and energetics of the ArnO− clusters for n=1,…,13 have been modeled in the framework of Diffusion Monte Carlo (DMC), using two- and three-body ab initio determined potentials derived previously by Buchachenko et al. [J. Chem. Phys. 112, 5852 (2000)], and Jakowski et al. [preceding paper, J. Chem. Phys. 118, 2731 (2003)], respectively. The anion cluster structures are largely determined by the two-body potential since the dominant contribution to the stabilization energy is due to pair interactions. However, the three-body effects are important since their role grows with n, from a few percent for n=2 to ca. 30% for n=12. The three-body effects are well approximated by the induction component only. The exchange and dispersion three-body and the induction many-body effects were found to be much less important. The effect of the spin–orbit coupling on the stabilization energies is small and almost independent of the size of a cluster. Specifically, it amounts to about 5% for ArO−, and to 0.1% for Ar12O−. The ab initio cluster stabilization energies are compared with those derived from the experimental measurements of electron detachment energies. The agreement is qualitatively good, and the origins of quantitative discrepancies are discussed.

Influence of lattice parameter scaling on local electronic and magnetic properties in La2CuO4

A large amount of static and dynamic NMR experiments have been carried out to determine the spin properties in high temperature superconductors cuprates (HTSC). In the majority of cuprates the magnetic shift at the planar copper nucleus is independent of the temperature when the field is applied perpendicular to the CuO2 planes and does not exhibit the expected reduction of the spin susceptibility in the superconducting state. This has been explained by a coincidental cancellation of on-site and transferred hyperfine fields at the copper nucleus, which would lead to a vanishing spin shift. To examine this explanation we determine the hyperfine fields by ab initio cluster calculations for La2CuO4 within the framework of spin-polarized density functional theory. In particular we investigate the sensitivity of the hyperfine fields on the lattice constants.

Cluster Calculations of Electric-Field-Gradients at the Ta Site for the Ferroelectric LiTaO 3 Crystal

Ab initio cluster calculations of electric-field-gradients at the Ta site in the ferroelectric LiTaO 3 crystal have been carried out for the temperature range 160 to 940 K. The calculations were made for the (TaO 6 ) m 7 cluster using the WT basis set for the tantalum and TZV basis set augmented with the polarization d -function (0.85) for the oxygen. The results of calculations of the V zz temperature dependence agreed well with experimental data.

Quantum Chemical Calculations on the Structure and Adsorption Properties of NO and N2O on Ag+ and Cu+ Ion-Exchanged Zeolites

Ab initio cluster quantum chemical calculations at the Hartree–Fock (HF/Lanl2dz) and correlated second-order Moller–Plesset perturbation theory (MP2/Lanl2dz) levels were performed for NO and N2O interactions with Ag+ and Cu+ ion-exchanged zeolites. The interaction energies were estimated in a conventional way and also corrected for basis set superposition errors. It was shown that the highly dispersed Ag+ counterions establish twofold coordination to the lattice oxygens on the zeolite surface, similar to the case of Cu+ ions. However, both NO and N2O bind relatively strongly to the Cu active sites of Cu+ ion-exchanged zeolites than those of the Ag+ site of the Ag+ ion-exchanged zeolites. Based on the results of these calculations, the two different forms of adsorption for these molecules on the catalyst surface, the nature of their binding and characteristics of the adsorption properties have been discussed. Finally, some comparisons with the results obtained by a variety of density functional theory calculations on target systems have been presented.

Cluster Ab Initio Calculations of the Shape of Local Potential Well and of Positions for Oxygen Atoms in PCN (C=Cd)

Potential relief and equilibrium positions for oxygen atoms in Cd--O--Nb, Cd--O--Cd and Nb--O--Nb chains are studied in Pb(Cd 1/3 Nb 2/3 )O 3 (PCN) and Ba(Cd 1/3 Nb 2/3 )O 3 (BCN). Total energy cluster calculations are performed using ab initio restricted Hartree-Fock method. The electron correlation contribution is calculated within Møller-Plesset method. It is revealed that in PCN the oxygen atoms in Cd--O--Cd and Cd--O--Nb chains move in multi-well potentials with four minima shifted transversely to fourfold [001] axis. Whereas the oxygen atom in Nb--O--Nb chain in PCN and all oxygen atoms in BCN move in single-well potentials.

Relaxation and electronic structure of the V 2O 3(0001) surface: ab initio cluster model studies

The electronic structure and geometric relaxation of the (0001) surface of rhombohedral vanadium sesquioxide, V 2O 3, is studied theoretically with large surface cluster models where ab initio density functional theory is used to characterize charging and bonding. Geometric relaxation in the topmost surface region, up to 5 layers, with its three different bulk terminations is determined by minimizing total energies of the clusters. This yields major relaxation effects depending on the termination. The oxygen layer termination OVV ′ exhibits strong relaxation of sub-surface vanadium layers resulting in increased ionic charging at the surface (measured by corresponding atom charges). The metal layer termination VV ′O leads to inwards relaxation of the two topmost vanadium layers by over 40% resulting also in increased surface charging. Ionic charging at the surface is the smallest for the half metal layer V ′OV termination where only the topmost vanadium layer relaxes inwards by 30% in addition to some rearrangement of sub-surface vanadium. This termination is believed to be the most stable of the three relaxed bulk-type terminations based also on analogies with experiments for Cr 2O 3(0001). However, total density-of-states and atom-projected partial densities-of-states curves depend relatively little on surface termination to allow a clear discrimination which could assist an unambiguous experimental identification.

Hydrogen adsorption on the indium-rich indium phosphide (001) surface: a novel way to produce bridging In-H-In bonds.

The indium phosphide (001) surface provides a unique chemical environment for studying the reactivity of hydrogen toward the electron-deficient group IIIA element, indium. Hydrogen adsorption on the In-rich delta(2 x 4) reconstruction produced a neutral, covalently bound bridging indium hydride. Using vibrational spectroscopy and ab initio cluster calculations, two types of bridging hydrides were identified, a (mu-H)In(2) and a (mu-H)(2)In(3) "butterfly-like" structure. These structures were formed owing to the large thermodynamic driving force for adsorption of H atoms on solid-state indium dimers.

Laser-Induced Desorption of CO from Cr2O3 (0001): Ab Initio Calculation of the Four-Dimensional Potential Energy Surface for an Intermediate Excited State

Ab initio cluster calculations for an excited state of the system CO/Cr2O3 (0001) are presented. A CO(a 3Π)-like state generated by an internal electronic excitation of the CO molecule (5σ → 2π*) is considered to be a possible intermediate for the laser-induced desorption of CO from Cr2O3 (0001). A four-dimensional potential energy surface (PES) for this state is calculated at the CASSCF level of theory. A few possible alternatives for the desorption intermediate are also discussed. The interaction mechanism between the excited CO molecule and the surface is analyzed, and it is shown that the change of symmetry of the quadrupole moment of the CO molecule is responsible for the topology of the calculated PES.

Chemisorption and Reactivity of Methanol on MgO Thin Films

Methanol adsorption on MgO thin films has been studied by Fourier transform infrared (FTIR) and thermal desorption spectroscopies (TDS), and by ab initio cluster model calculations. Depending on the preparation conditions, films with various concentrations of defects have been obtained. These films exhibit different reactivity toward adsorbed methanol. In particular, at temperatures of 580 K, H2 is released from defect-rich films while no production of hydrogen is observed on defect-poor films. The calculations show that methanol can interact in three different ways with the surface giving rise to physisorption, chemisorption, or heterolytic dissociation into CH3O- and H+ fragments depending on the adsorption site. Chemisorption and dissociation occur only at defect sites, like low-coordinated ions at steps. Oxygen vacancies (F centers) are proposed as the sites present on the defect-rich films which are responsible for hydrogen release at high temperature.

Cluster size convergent full relativistic density-functional calculations of single atom adsorption

Within full relativistic four-component ab initio density functional cluster calculations we are now able to present results for the adsorption energy and bond distance of single atoms on surfaces. In order to compare with other (non-relativistic) calculations we have chosen Cu on a Cu(100) surface as an example. The convergence with respect to the cluster size is achieved with 56 atoms. This demonstrates that even for good conductors convergent molecular cluster calculations with reliable results are possible.