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For the first time, the dependences of the strain energy and the band gap of achiral nanotubes obtained by folding monolayers of gallium(II) sulfides and selenides on their diameter have been calculated nonempirically. The calculations were performed using the CRYSTAL17 software package using an atomic basis set within the hybrid density functional theory with a thirteen-percent Hartree–Fock exchange. To take into account the dispersion interactions between the layers in the crystal, the Grimme empirical correction is included in the calculations. As a result of simulation of nanotubes with different chirality and different diameters, the minimum diameters of single-walled nanotubes, at which the integrity of chemical bonds on their outer surface is preserved, are established. It is shown that the dependence of the strain energy on the diameter satisfies the classical law of inverse squares and is the same for nanotubes of the zigzag and armchair types.

We used hybrid density functional theory to investigate 50% cubic solid solutions based on Fe-doped LaGaO3 and LaAlO3. The supercell composed of 8 primitive cells was used. All the possible configu...

A force field is proposed that reproduces with a high accuracy a large number of properties of the bulk crystal MoS2 phases, monolayers, and nanotubes. The reproduced values are both the experimental results and the results of quantum chemical calculations. The elaborated interaction potential can be useful primarily for investigation of multiwall MoS2 nanotubes and their thermodynamic properties, especially, since the potential is able to reproduce the frequencies of the crystal phonon spectrum. In this study the proposed potential is applied to simulate the temperature dependence of a number of properties of the armchair and zigzag nanotubes. The calculations have been performed using molecular mechanics method within the framework of quasi harmonic approximation which is carried out through the estimation of the temperature dependence of the Helmholtz free energy.

A new analytical expression was derived for the chemical potential of a hard sphere dipole in hard sphere dipole fluid at infinite dilution of the solute using the mean spherical approximation (MSA). A set of Monte Carlo (MC) simulations has been carried out to investigate the scope of applicability of the derived equation. The mean reaction field (MRF) approach was used in our MC computations. Two different MC methods (Widom particle insertion and thermodynamic integration) were applied for obtaining the chemical potential change associated with the dipole creation at the solute particle to provide adequate accuracy of the MC simulations. Also, corresponding changes in the mean potential energy were calculated by direct method and by thermodynamic integration. The solvation energies have been obtained for the systems of dipolar hard spheres with reduced dipole moment 1.0 at the reduced densities 0.2, 0.5, and 0.8. Computations have been made for solute particles with the reduced dipole moment varied from 0.0 to 1.5 and the hard sphere diameter varied from 0.5 to 2.0. The variation of those quantities with the molecular parameters was analyzed and compared with the MSA equation and Kirkwood classical expressions. It was found that the MSA calculations agree relatively well with MC simulations at densities less than 0.5 and solute dipole moment less than 1.0.

An increasing number of industrial applications rely on controlling solutes in water above and below its critical point. Processes such as hydrothermal synthesis, steam power generation and ultra-high enthalpy geothermal power are all influenced by factors such as mineral precipitation, pH and solute speciation. The supercritical point of water is remarkable in that slight changes in temperature and pressure can cause dramatic changes in some solute properties. Here, it was found that our approach reliant on molecular statistical thermodynamic expressions for hard sphere (HS), ion-dipole and dipole-dipole interactions via mean spherical approximation (MSA) provided excellent agreement to available experimental data. In addition to model parameters having some physical meaning, this approach used less adjustable parameters than the well-known Helgeson-Kirkham-Flowers (HKF) model. Furthermore, the model was used to obtain standard thermodynamic values for HCl0(aq), KCl0(aq) and NaOH0(aq) ion pairs. In total, modeling parameters for 10 different aqueous species were obtained to demonstrate the capabilities of the approach.

For the first time, the structural and stability properties of multi-walled GaS, GaTe, and Janus Ga2STe nanotubes have been simulated using molecular-mechanics calculations. The armchair and zigzag nanotubes with different wall numbers and diameters have been considered. The force field used for GaS, GaTe and Ga2STe multi-walled nanotubes is based on the interaction potentials recently developed to describe monolayers of the transition and post-transition metal chalcogenides. The Buckingham potentials have been introduced to model the interlayer and interwall interactions. The results of quantum mechanical calculations for a number of bulk crystals, mono and bilayers, as well as small diameter single-wall nanotubes have been employed to check the validity of the proposed force field. With the help of an adapted force field, large eight-walled nanotubes with an outer diameter of 32 nm were simulated, which at present cannot be studied using the first principles of quantum mechanical methods. Molecular mechanics simulations predict the faceted structure of armchair GaS, GaTe, and Ga2STe multi-walled nanotubes with a sufficiently large diameter and a cylindrical structure for all zigzag nanotubes. The Janus nanotubes' stability is superior to that of binary nanotubes and may probably achieve a maximum at certain wall numbers and diameters.

The properties of multi-walled MoS2 nanotubes have been investigated by the first principles calculations and by molecular mechanics (MM) simulations using a revised three-body force field. The density functional theory (DFT) calculations have been performed on single-, double- and triple-walled MoS2 nanotubes. The new version of the force field is able to reproduce the structure integrity of the MoS2 nanotubes at temperatures up to 700 K through the molecular dynamics simulations. Comparison of the results of first principles and MM simulations of the multi-walled nanotubes demonstrates satisfactory agreement. The results of DFT and MM simulations indicate that the difference between chirality indices of adjacent shells of a multi-walled nanotube is the main factor that determines a possibility of the nanotube to be synthesized. The structure of zigzag 12-walled nanotubes with chirality indices difference 12 and 13, simulated by MM method and using the proposed force field, is the most close to the structure of experimentally detected nanotubes.

Temperature dependence of the Young’s modulus of cylindrical single-wall nanotubes with zigzag and armchair chiralities and consolidated-wall nanotubes has been studied by the molecular mechanics method with the use of the atom–atom potential. The nanotubes have been obtained by rolling up of crystal layers (111) of TiO2 with fluorite structure. Calculations have been performed for isothermal conditions on the basis of calculating the Helmholtz free energy of the system. The dependence of the Helmholtz free energy of nanotubes on the period has been calculated in the quasi-harmonic approximation as a result of calculation of phonon frequencies. It has been shown that the temperature dependence of the stiffness of nanotubes is determined by their chirality, and some nanotubes exibit anomalous behavior of both the Young’s modulus and the period of unit cell with variation in temperature.

Adsorption of Rb+ to the quartz(101)–aqueous interface at room temperature was studied with specular X-ray reflectivity, resonant anomalous X-ray reflectivity, and density functional theory. The interfacial water structures observed in deionized water and 10 mM RbCl solution at pH 9.8 were similar, having a first water layer at height of 1.7 ± 0.1 A above the quartz surface and a second layer at 4.8 ± 0.1 A and 3.9 ± 0.8 A for the water and RbCl solutions, respectively. The adsorbed Rb+ distribution is broad and consists of presumed inner-sphere (IS) and outer-sphere (OS) complexes at heights of 1.8 ± 0.1 and 6.4 ± 1.0 A, respectively. Projector-augmented planewave density functional theory (DFT) calculations of potential configurations for neutral and negatively charged quartz(101) surfaces at pH 7 and 12, respectively, reveal a water structure in agreement with experimental results. These DFT calculations also show differences in adsorbed speciation of Rb+ between these two conditions. At pH 7, the lowe...

Hybrid density functional theory calculations are performed for the first time to compare the stability, structural and electronic properties of monolayers and single-wall nanotubes based on pure Mo(W)S(Se)2 and mixed (Janus) Mo(W)SSe dichalcogenides. The stability, structural and electronic properties of Mo and W dichalcogenide nanotubes have been compared at different wall compositions, chiralities and diameters using the same calculation scheme. Different types of mixed nanotubes are considered – with S or Se atoms on the outer (inner) shell of the nanotube. It was found that nanotubes Se(out)WS(in) with average diameter (Davr) greater than ≈40 A have the negative strain energy. Our calculations show that the band gap is direct for zigzag MS2 and S(out)MSe(in) nanotubes (M = Mo, W) but it becomes indirect in armchair nanotubes. For the MSe2 and Se(out)MS(in) nanotubes of both chiralities, the band gap is mostly direct, except the armchair tubes with Davr

We have performed first-principles calculations of the atomic and electronic structures and stability of three (α, β and γ) layered V 2 O 5 phases and their free layers within the same computational approach. In computations, we have used a hybrid exchange–correlation functional within the density functional theory and a basis set of localized atomic orbitals. All the lattice parameters and the atomic positions have been totally optimized and the phonon frequencies at the Γ-point of the Brillouin zone have been obtained. The calculated relative stability of considered bulk phases decreases as α > γ > β. The calculated relative stability of layers differs from that of bulk phases and decreases as β > α ⩾ γ. The possibility of the folding of V 2 O 5 layers into nanotubes is discussed.

A novel mechanism for protonating bridging O atoms (Obr) and dissolving silica is proposed that is consistent with experimental data and quantum mechanical simulations of the α-quartz (101)/water i...

The line symmetry groups for one-periodic (1D) nanostructures with rotohelical symmetry have been applied for symmetry analysis of double-wall boron nitride and titania nano- tubes (DW BN and TiO2 NTs) formed by rolling up the stoichiometric two-periodic (2D) slabs of hexagonal structure with the same or opposite orientation of translation and chiral vectors. We have considered the two sets of commensurate DW BN and TiO2 NTs with either armchair- or zigzag-type chiralities, i.e., (n1,n1)@(n2,n2 )o r (n1,0)@(n2,0), respectively. To establish theequilibriuminterwalldistancescorrespondingtotheminimaof energy, we have varied chiral indices n1 and n2 of the constituent single-wall (SW) nanotubes. To analyze the structural and elec- tronic properties of hexagonal DW NTs, we have performed ab initio LCAO calculations using the hybrid HartreeFock/KohnSham exchange-correlation functional PBE0 as implemented in CRYSTAL-09 code. The inversely stacked structure of zigzag-type DW BN NT, characterized by arrangement of positively and negatively charged rings in each atomic cross section (consisting of either B or N atoms, respectively), has been found to be energetically more preferable as compared to the straightly stacked structure containing nanotube rings consisting of the same type ofatomsincrosssections,i.e.,B(N)andB(N).Inarmchair-typeDWBNNTs,eachatomicringcontainsthewholenumberofBN bonds,whichreducestheelectrostaticinteractionbetweenbothwalls.Ontheotherhand,maincontributiontointerwallbondingin DW TiO2 NTs is provided by interaction between the nearest oxygen and titanium ions of neighboring shells. The interaction between the walls results in a decrease of band gaps for double-wall NTs as compared to those for SW NTs, which is substantially larger for TiO2.

Adsorption of Zn2+ at the rutile TiO2 (110)-aqueous interface was studied with Born–Oppenheimer molecular dynamics at 300 K. Simulations were carried out using the periodically repeated slab model with vacuum gap of 15 A filled with 72 H2O molecules. Two possible adsorption sites, monodentate above bridging oxygen (Ti–O–Ti or Obr) and bidentate above terminal oxygens (Ti–O), were investigated. Sites with different coordination environment for adsorbed Zn2+ differ from each other by the position of Zn2+ above surface level and by characteristic Zn–O distances. Obtained results gave evidence that 4-fold coordination of adsorbed Zn2+ is more probable than the 6-fold coordination found for aqueous species. The hydrolysis of H2O molecules was observed in the first coordination shell of adsorbed ion, resulting in formation of OH– groups attached to Zn2+. Calculated energies favor the tetrahedral bidentate structure of hydrated Zn2+ on the rutile surface. The model structures are compared to observed positions o...

Plane wave density functional theory (DFT) calculations have been performed to study the atomic structure, preferred H2O adsorption sites, adsorption energies, and vibrational frequencies for water adsorption on the α-quartz (101) surface. Surface energies and atomic displacements on the vacuum-reconstructed, hydrolyzed, and solvated surfaces have been calculated and compared with available experimental and theoretical data. By considering different initial positions of H2O molecules, the most stable structures of water adsorption at different coverages have been determined. Calculated H2O adsorption energies are in the range −55 to −65 kJ/mol, consistent with experimental data. The lowest and the highest O−H stretching vibrational bands may be attributed to different states of silanol groups on the water-covered surface. The dissociation energy of the silanol group on the surface covered by the adsorption monolayer is estimated to be +80 kJ/mol. The metastable states for the protonated surface bridging O...

LCAO calculations have been used to study atomic and electronic structure of the oxygen vacancy at (001) ZrO2 termination of the cubic phase of Y-doped BaZrO3.By considering different positions of Y substituents, the vacancy formation energies are calculated relative to the stable phases of ZrO2 and Y2O3. It is demonstrated that the surface vacancy stability can be affected by the location depth of the nearest Y dopant atoms in the top surface layers. The calculated surface migration energy of 0.3 eV is small and thus we expect surface processes active already at moderate temperatures. The vacancy formation energy on the surface (1.3 eV) is much smaller than in the bulk (3.3 eV); thus we predict strong defect segregation towards the surface.

For ab initio simulations on hexagonal single-wall BN and TiO2 nanotubes (SW NTs), we have applied the formalism of line symmetry groups describing one-periodic (1D) nanostructures with rotohelical symmetry. Both types of NTs can be formed by rolling up the stoichiometric nanosheets of either (i) a (0001) monolayer of BN hexagonal phase or (ii) a three-layer (111) slab of fluorite-type TiO2 phase. Optimized parameters of the atomic and electronic structure of corresponding slabs and nanotubes have been calculated using hybrid LCAO method as implemented in CRYSTAL code. Their band gaps (Δegap) and strain energies (Estrain) have been analyzed as functions of NT diameter (DNT). For hexagonal BN and TiO2 nanotubes, certain qualitative similarities between the Δegap(DNT) or Estrain(DNT) functions exist despite the different chemical nature.

The results of the first-principles simulations of water adsorption on (001) surface of both cubic and orthorhombic phases of SrHfO3 crystal are presented. The dissociative and molecular water adsorption is investigated. The role of geometrical and chemical-bonding factors in the relative stability of different adsorption structures is discussed. A symmetry reducing from cubic to orthorhombic is shown to provide more optimal mutual orientation of adsorbate species thus increasing the water adsorption energy on the orthorhombic crystal compared with the cubic one.

We study the relation between the hydrogen bonding and the vibrational frequency spectra of water on the (110) surface of rutile (α-TiO2) with three structural layers of adsorbed water. Using ab initio molecular dynamics simulations at 280, 300, and 320 K, we find strong, crystallographically controlled adsorption sites, in general agreement with synchrotron X-ray and classical molecular dynamics simulations. We demonstrate that these sites are produced by strong hydrogen bonds formed between the surface oxygen atoms and the sorbed water molecules. The strength of these bonds is manifested by substantial broadening of the stretching mode vibrational band. The overall vibrational spectrum obtained from our simulations is in good agreement with inelastic neutron scattering experiments. We correlate the vibrational spectrum with different bonds at the surface to transform these vibrational measurements into a spectroscopy of surface interactions.

The study of the bulk and surface properties of cubic (fluorite structure) HfO 2 and UO 2 was performed using the hybrid Hartree–Fock density functional theory linear combination of atomic orbitals simulations via the CRYSTAL06 computer code. The Stuttgart small-core pseudopotentials and corresponding basis sets were used for the core–valence interactions. The influence of relativistic effects on the structure and properties of the systems was studied. It was found that surface properties of Mott–Hubbard dielectric UO 2 differ from those found for other metal oxides with the closed-shell configuration of d -electrons.

Hybrid HF-DFT LCAO simulations of SrZrO3 and SrTiO3(001) surface properties are performed in a single-slab model framework. The SrZrO3(001) surface was studied by an ab initio method for the first time. Three slab models with different surface terminations including up to 8 atomic planes were used for calculation of the various surface characteristics (surface energies, atomic charges, density of electronic states). The dependence of the results on the chosen model and on the kind of d -element is analyzed. The dissimilarity in the surface oxygen atom contributions to the total density of states of two crystals is attributed to the more ionic nature of Zr–O bonds compared to Ti–O bonds. It is found that in the case of SrZrO3 the electronic density is biased towards the SrO-terminated surface and this surface should be more basic in nature than the SrO surface of SrTiO3 crystal. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The density functional LCAO calculations using DFT (PBE) and hybrid HF-DFT (B3LYP) functionals are presented for the electronic and structural properties of bare and hydroxylated low index surfaces of SnO2 crystal. The dissociative and associative water adsorption on (110) and (100) SnO2 surfaces is investigated. The role of geometrical and chemical-bonding factors in the relative stability of different adsorption structures is discussed. The comparison of plain DFT functionals with the B3LYP hybrid functional shows that, in contrast to former DFT plane-wave calculations (spontaneous dissociation), an associative adsorption of the water molecules becomes possible not only in the case of the (100) surface but also at the most stable (110) surface. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The authors thank A Gulans, B Polyakov and S Vlassov for stimulating discussions. This study has been supported by the ERA.Net RUS Plus project No. 237 Watersplit. AB, RE and SL acknowledge the financial support by the Russian Foundation for Basic Research (Grant No. 17-03-00130-a) and the assistance of the Saint Petersburg State University Computer Center in the accomplishment of high-performance computations., Morphologically reproducible wurtzite-structured zinc oxide nanowires (ZnO NWs) can be synthesized by different methods. Since ZnO NWs have been found to possess piezoelectricity, a comprehensive study of their mechanical properties, e.g. deformations caused by external compression or stretching, is one of the actual tasks of this paper. We have calculated wurtzite-structured [0 0 0 1]-oriented ZnO NWs whose diameters have been varied within 1–5 nm and 1–20 nm ranges when using either ab initio (hybrid DFT-LCAO) or force-field (molecular mechanical) methods, respectively (the minimum diameter dNW of experimentally synthesized NWs has been estimated on average to be ~20 nm). When using both chosen calculation approaches, the values of Young's moduli determined for the mentioned ranges of NW diameters have been found to be qualitatively compatible (168–169 GPa for 5 nm NW thickness), whereas results of molecular mechanical simulations on YNW for 20 nm-thick NWs (160–162 GPa) have been qualitatively comparable with those experimentally measured along the [0 0 0 1] direction of NW loading. In all the cases, a gradual increase of the NW diameter has resulted in an asymptotic decrease of Young's modulus consequently approaching that (Yb) of wurtzite-structured ZnO bulk along its [0 0 0 1] axis. The novelty of this study is that we combine the computation methods of quantum chemistry and molecular mechanics, while the majority of previous studies with the same aim have focused on the application of different classical molecular dynamical methods., ERA.Net RUS Plus project No. 237 Watersplit; Russian Foundation for Basic Research (Grant No. 17-03-00130-a); Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

The results of the density functional (DFT) LCAO and plane wave (PW) calculations of the electronic and structural properties of four known SrZrO3 phases (Pm3m, I4/mcm, Cmcm and Pbnm) are presented and discussed. The calculated unit cell energies and relative stability of these phases agree well with the experimental sequence of SrZrO3 phases as the temperature increases. The lattice structure parameters optimized in the PW calculations for all four phases are in good agreement with the experimental neutron diffraction data. The LCAO and PW results for the electronic structure, density of states and chemical bonding in the cubic phase (Pm3m) are discussed in detail and compared with the results of previous PW calculations. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Liu and co-workers [Phys. Rev. B 82, 161415 (2010)] discussed the long-standing debate regarding whether H${}_{2}$O molecules on the defect-free (110) surface of rutile (\ensuremath{\alpha}-TiO${}_{2}$) sorb associatively, or there is dissociation of some or all first-layer water to produce hydroxyl surface sites. They conducted static density functional theory (DFT) and DFT molecular dynamics (DFT-MD) investigations using a range of cell configurations and functionals. We have reproduced their static DFT calculations of the influence of crystal slab thickness on water sorption energies. However, we disagree with several assertions made by these authors: (a) that second-layer water structuring and hydrogen bonding to surface oxygens and adsorbed water molecules are ``weak''; (b) that translational diffusion of water molecules in direct contact with the surface approaches that of bulk liquid water; and (c) that there is no dissociation of adsorbed water at this surface in contact with liquid water. These assertions directly contradict our published work, which compared synchrotron x-ray crystal truncation rod, second harmonic generation, quasielastic neutron scattering, surface charge titration, and classical MD simulations of rutile (110) single-crystal surfaces and (110)-dominated powders in contact with bulk water, and (110)-dominated rutile nanoparticles with several monolayers of adsorbed water.

The electronic structures of four well-known modifications of crystalline SrZrO3 with different symmetries, namely, the cubic (Pm3m), tetragonal (I4/mcm), and two orthorhombic (Cmcm, Pbnm) modifications, are calculated in the framework of the density-functional theory in the basis set of the linear combination of atomic orbitals (LCAO). A comparative analysis of the electronic properties of the crystals under consideration is performed on the basis of the calculated band structures and densities of states (the total densities of states and the densities of states projected onto the atomic states). The calculated relative stabilities of the different modifications are in good agreement with the experimental data on the phase transitions in the SrZrO3 crystal: the low-temperature modifications with lower symmetry are more stable. The ionicities of chemical bonding in different modifications of crystalline SrZrO3 are compared by analyzing the Mulliken populations and constructing the localized Wannier functions for the occupied energy bands.

The molecular cluster approach to the problem of small-radius impurity centres in solids is discussed. It is shown that the molecular orbitals obtained for the cluster of host atoms can be connected with the crystalline states only for clusters having the form of a large unit cell. In this case the molecular orbitals of the cluster have the sense of Wannier functions, the appropriate one-electron energies the sense of mean values of the band energies in the large unit cell scheme of the crystalline state classification. It is found that the cluster model is realistic for solids with narrow (1 to 5 eV) energy bands. The convergence of the results with increasing cluster size is shown for lithium chloride ([LiCl6]5− [Li14Cl13]+, and [Li18, Cl18]0 are considered), the molecular cluster calculations are performed for some alkali halides and the results are compared with those obtained using band structure calculations. [Russian Language Ignored].