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Diamagnetism is the response of dynamical compositions of charged particles, electrons, and nuclei, to an incident magnetic field. In this paper, we study how the finite temperature and finite nuclear masses affect the diamagnetic susceptibilities of selected small atoms and molecules, as limiting cases of dilute gas. We use nonrelativistic path-integral Monte Carlo simulation (PIMC), where both electrons and nuclei are treated on equal footing at finite temperatures in sampling exact Coulomb pair density matrices. The PIMC estimator of diamagnetic susceptibility has been briefly introduced in Ceperley [D. M. Ceperley, Rev. Mod. Phys. 67, 279 (1995)], but here we present a comprehensive derivation, discussion of practical effects, and proof-of-concept results for selected few-body Coulomb systems. Our results are in perfect agreement with high-accuracy literature references, where available, but also demonstrate additional thermal effects of the diamagnetic response of low-mass systems. peerReviewed

Electronic structure calculations are mostly carried out with Coulomb potential singularity adapted basis sets such as STO or contracted GTO. With another basis or for heavy elements, the pseudopotentials may appear as a practical alternative. Here, we introduce the exact pseudopotential (EPP) to remove the Coulomb singularity and test it for orbitals of small atoms with the interpolating wavelet basis set. We apply EPP to the Galerkin method with a basis set consisting of Deslauriers–Dubuc scaling functions on the half-infinite real interval. We demonstrate the EPP–Galerkin method by computing the hydrogen atom 1s, 2s, and 2p orbitals and helium atom configurations He1s2, He1s2s1S, and He1s2s3S. We compare the method to the ordinary interpolating wavelet Galerkin method (OIW–Galerkin), handling the singularity at the nucleus by excluding the scaling function located at the origin from the basis. We also compare the performance of our approach to that of finite-difference approach, which is another practical method for spherical atoms. We find the accuracy of the EPP–Galerkin method to be better than both of the above-mentioned methods.

The article gives two examples of the incorporation of liquid phase coated low refractive index (RI) and high refractive index materials in a proof-of-concept optical devices. Using ultra low RI (1.25) and ultra high RI (2.05), a 12-layer dielectric mirror is constructed. Optical properties of the stack are compared against to simulations, and a SEM-image of the stack is provided. The second example of using of high RI/low RI materials is provided where UV-curable, high RI material (1.9) nanoimprint material is nanopatterned, followed by gap-fill and protections by a cover glass using a dual cure (UV+thermal) RI 1.4 low refractive index bonding adhesive.

Utilization of the renewable energy sources is one of the main challenges in the state-of-the-art technologies for CO recycling. Here we have taken advantage of the solar light harvesting in the thermocatalytic approach to carbon dioxide methanation. The large-surface-area Ni/CeO catalyst produced by a scalable low-cost method was characterized and tested in the dark and under solar light irradiation conditions. Light-assisted CO conversion experiments as well as in-situ DRIFT spectrometry, performed at different illumination intensities, have revealed a dual effect of the incident photons on the catalytic properties of the two-component Ni/CeO catalyst. On the one hand, absorbed photons induce a localized surface plasmon resonance in the Ni nanoparticles followed by dissipation of the heat to the oxide matrix. On the other hand, the illumination activates the photocatalytic properties of the CeO support, which leads to an increase in the concentration of the intermediates being precursor for methane production. Analysis of the methane production at different temperatures and illumination conditions has shown that the methanation reaction in our case is controlled by a photothermally-activated process. The used approach has allowed us to increase the reaction rate up to 2.4 times and consequently to decrease the power consumption by 20 % under solar illumination, thus replacing the conventional thermal activation of the reaction with a green energy source., This work has been done in the framework of the doctorate programme in Materials Science of the Autonomous University of Barcelona. V. Golovanova has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 754397. Authors acknowledge Generalitat de Catalunya for financial support through the CERCA Programme, M2E (2017 SGR 1246) and the CSC – IT Center for Science, Espoo, Finland for provided resources. IREC also acknowledges additional support by the European Regional Development Funds (ERDF, FEDER) and by MINECO coordinated projects MAT2014-59961-C2, ENE2016-80788-C5-5-R and ENE2017-85087-C3. M. C. Spadaro and J. Arbiol acknowledge funding from Generalitat de Catalunya 2017 SGR 327. ICN2 is supported by the Severo Ochoa programme from Spanish MINECO (Grant No. SEV-2017-0706) and is funded by the CERCA Programme / Generalitat de Catalunya. M. C. Spadaro has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 754510 (PROBIST) and the Severo Ochoa programme.

This rapid communication gives the salient points and results of the theoretical investigation of a chemical reaction for efficient selective hydrogen production. The clean fuel produced is a sustainable energy source. Accurate methods based on quantum theory are used because the changing electronic structure is a probe that monitors reactions. The reaction between water and carbon monoxide is used industrially with metal catalysts, usually platinum. There is a considerable economic and environmental challenge underpinning this fundamental investigation where bond dissociation plays an essential role. A bond dissociation process is often the limiting step of reaction rates for industrial catalysis. Most mainstream quantum approaches fail to a greater or lesser degree in the description of this process. The present work advocates a promising alternative: the initial analysis of statistical data generated by the Quantum Monte Carlo (QMC) method demonstrated very stringent statistical accuracy for essential information on hydrogen production via the water-gas shift reaction with platinum catalyst. The transition state structure is obtained from QMC force constants and illustrated here. It corresponds to water OH-stretch concerted with Pt-H bond formation, whilst the OH oxygen atom begins to interact with the CO carbon. The present QMC evaluation of the corresponding activation barrier is low: 17.0 ± 0.2 kcal/mol. It is close to the experimental apparent activation energy of 17.05 kcal/mol. This method is applicable to a wide range of similar systems. acceptedVersion

Quantum Monte Carlo (QMC) methods offer ab initio calculations of remarkable accuracy for reaction activation barriers (to 1 kJ/mol or 0.01 eV), even at solid surfaces. Chemical reactions generally involve bond dissociation that is poorly described by Hartree-Fock and density functional theory (DFT) methods. However, such processes are often the key to rate-limiting reaction steps at solid surfaces. In this work, a novel QMC methodology is demonstrated as a solution. Carbon monoxide (CO) on platinum reacts with water, first giving the addition products. QMC methods are used here to investigate addition at close-packed Pt(111). The CO is preadsorbed on Pt(111). First, an approaching water molecule is only partially dissociated. At the same time, its oxygen atom binds to CO giving an adsorbed formate moiety (COOH) and Pt-H. This concerted addition is rate-limiting. The QMC activation barrier (in kJ/mol) is 71.0 ± 0.7, and its apparent measured value is 71.4. This showcases QMC as a method to investigate related catalytic systems. acceptedVersion

The path integral formulation of quantum mechanics, i.e., the idea that the evolution of a quantum system is determined as a sum over all the possible trajectories that would take the system from the initial to its final state of its dynamical evolution, is perhaps the most elegant and universal framework developed in theoretical physics, second only to the standard model of particle physics. In this Tutorial, we retrace the steps that led to the creation of such a remarkable framework, discuss its foundations, and present some of the classical examples of problems that can be solved using the path integral formalism, as a way to introduce the readers to the topic and help them get familiar with the formalism. Then, we focus our attention on the use of path integrals in optics and photonics and discuss in detail how they have been used in the past to approach several problems, ranging from the propagation of light in inhomogeneous media to parametric amplification and quantum nonlinear optics in arbitrary media. To complement this, we also briefly present the path integral Monte Carlo method, as a valuable computational resource for condensed matter physics, and discuss its potential applications and advantages if used in photonics. publishedVersion

We present a new approach based on real time domain Feynman path integrals (RTPI) for electronic structure calculations and quantum dynamics, which includes correlations between particles exactly but within the numerical accuracy. We demonstrate that incoherent propagation by keeping the wave function real is a novel method for finding and simulation of the ground state, similar to Diffusion Monte Carlo (DMC) method, but introducing new useful tools lacking in DMC. We use 1D Hooke’s atom, a two-electron system with very strong correlation, as our test case, which we solve with incoherent RTPI (iRTPI) and compare against DMC. This system provides an excellent test case due to exact solutions for some confinements and because in 1D the Coulomb singularity is stronger than in two or three dimensional space. The use of Monte Carlo grid is shown to be efficient for which we determine useful numerical parameters. Furthermore, we discuss another novel approach achieved by combining the strengths of iRTPI and DMC. We also show usefulness of the perturbation theory for analytical approximates in case of strong confinements. acceptedVersion

The RLS trade-off of EUV resists has been a major technical issue for high-volume manufacturing using EUVL. Significant attempts to develop of chemically-amplified resists, metal-containing resists, and a variety of other material classes have been made to obtain low LER at high resolution (R) and at a reasonable sensitivity (S). Previously, we have developed and reported work on silanol-containing polyhydrogensilsesquioxane resins and their use as negative tone resists. The developed silanol-containing polymer resists have demonstrated enhanced EUV sensitivity compared to traditional hydrogen silsesquioxane resins, and at the same time maintaining excellent etch properties. The resist may enable a bilayer stack technology in EUVL. Herein we report novel functionalized polyhydrogensilsesquioxane polymers and their use as negative tone resists. These materials exhibit improved LER/LWR and reasonably good EUV sensitivity. In best cases, data suggests no residues or bridging in the non-exposed areas. The optimized resist exhibits sub-20nm halfpitch resolution, low LER (2-3nm), and reasonable sensitivity (82.5 mJ/cm2). In addition, we also investigated the effect of three organic underlayers for EUV patterning and compared with the silicon substrate.

Macrocyclic dyes such as phthalocyanine and porphyrin molecules are modeled on (10110) wurzite surfaces using the DFT and molecular dynamics approaches. It is found that the single dye anchored on the wurtzite surface stabilizes in an inclined geometry with its core facing the surface at a tilting angle of ca 60 ∘ . The tilting of the dye relative to the crystal surface has a dual effect on the charge transfer from a chromophore to the semiconductor. Increasing the tilting angle leads to a stronger coupling between the lowest level of the semiconductor conduction band and dye’s LUMO, thus raising the tunneling probability of the electron injection. By contrast, the electrostatic interaction between units upon the tilting of macrocycles results in a lowering of the molecule LUMO level with respect to the conduction band minimum of the wurzite crystal, which may hinder the electron transfer. The type of a linker and peripheral substituents significantly affect the mutual conformation of the moieties, and their proper choice can facilitate the photoinduced charge transfer reactions. publishedVersion

First-principles density functional theory calculations in the generalized gradient approximation, with plane wave basis set and pseudopotentials, have been used to investigate the desorption pathways of molecular oxygen species adsorbed on the SnO2 (110) surface. Energetics of the thermodynamically favored precursors is studied in dependence on the surface charge provided either by surface defects or by donor type impurities from the near-surface region. The resonant desorption modes of O2 molecules are examined in the framework of ab initio atomic thermodynamics and relationship of these results to experimental observations is discussed. acceptedVersion

A p-type dye-sensitized solar cell, where the dye injects a hole into the semiconductor, could be combined with a typical Gratzel cell to create an efficient tandem device. However, the current p-type devices suffer from low efficiency. Here, geometries and electronic structures of four perylenemonoimide-based dyes (1–4) both as free and adsorbed on the NiO(100) semiconductor surface have been investigated to gain a better understanding of the p-type devices. In particular, the electronic transitions relevant to charge transfer between the dye and the surface have been identified. Excitations have been evaluated using the time-dependent DFT calculations, and the roles of frontier orbitals and band edges in transitions have been assessed. The adsorbed dyes can adopt either upright or slightly tilted geometries depending on the structure of the anchoring group and the binding mode of the dye. The adsorption slightly lowers the NiO band gap, from 4.06 eV to 3.90–3.96 eV, depending on the surface–adsorbate system and the band gaps of the dye molecules by 0.1–0.2 eV. Additionally, the adsorption mode of dye 1 moves the LUMO+1 level down by 0.5 eV. The effective mass of charge carrier holes is significantly smaller at the NiO surface than in the bulk indicating the importance of surface conductivity. We also found that the potential drop, i.e. the driving force for charge transfer from NiO to the dye molecule, depends on the adsorption mode of 1.

We demonstrate computation of total dynamic multipole polarizabilities using path-integral Monte Carlo method (PIMC). The PIMC approach enables accurate thermal and nonadiabatic mixing of electronic, rotational, and vibrational degrees of freedom. Therefore, we can study the thermal effects, or lack thereof, in the full multipole spectra of the chosen one- and two-electron systems: H, Ps, He, Ps2, H2, and HD+. We first compute multipole-multipole correlation functions up to octupole order in imaginary time. The real-domain spectral function is then obtained by analytical continuation with the maximum entropy method. In general, sharpness of the active spectra is limited, but the obtained off-resonant polarizabilities are in good agreement with the existing literature. Several weak and strong thermal effects are observed. Furthermore, the polarizabilities of Ps2 and some higher multipole and higher frequency data have not been published before. In addition, we compute isotropic dispersion coefficients C6, C8, and C10 between pairs of species using the simplified Casimir-Polder formulas. acceptedVersion

The system of two interacting electrons in one-dimensional harmonic potential or Hooke's atom is considered, again. On one hand, it appears as a model for quantum dots in a strong confinement regime, and on the other hand, it provides us with a hard test bench for new methods with the "space splitting" arising from the one-dimensional Coulomb potential. Here, we complete the numerous previous studies of the ground state of Hooke's atom by including the excited states and dynamics, not considered earlier. With the perturbation theory, we reach essentially exact eigenstate energies and wave functions for the strong confinement regime as novel results. We also consider external perturbation induced quantum dynamics in a simple separable case. Finally, we test our novel numerical approach based on real-time path integrals (RTPIs) in reproducing the above. The RTPI turns out to be a straightforward approach with exact account of electronic correlations for solving the eigenstates and dynamics without the conventional restrictions of electronic structure methods. acceptedVersion

Extreme ultraviolet lithography (EUVL, λ = 13.5 nm) is the most promising candidate to pattern the finest features in the next-generation integrated circuit manufacturing. Chemically-amplified resists (CARs) have long been used as state-of-the art photoresists and have been considered as EUV resist. Recently, inorganic and metal-containing resist materials have received significant attention in both academia and industry areas, with the aim to improve the resist performance in terms of resist resolution (R), line-edge roughness (LER), and sensitivity (S) to solve the well-known RLS trade-off. However, the resists reported to date usually have either problem in terms of RLS trade-off or pose metal contamination, which is a serious issue in expensive EUV equipment. Differently, in this report, we demonstrate our recent success in the development of the photochemistry of silicon compounds and resist formulations to obtain novel EUV negative tone resists with high resolution (up to 22nm pitch line/space patterns), low line-edge roughness (1-3nm) with reasonable EUV sensitivity. We also discuss their high etch selectivity to a PiBond’s SOC organic underlayer, which enable a bilayer lithography stack for EUVL patterning. Their excellent etch performances by RIE plasma is also reported.

A phthalocyanine molecule adsorbed on the (1010) surface of wurtzite CdSe is theoretically modeled by the DFT method. We have found that a linker does not affect substantially the redox properties of phthalocyanine, while saturation of the macrocycle with peripheral substituent groups causes a downward shift in the energy position of its frontier orbitals that can hinder electron injection to the CdSe surface. Tilting of the phthalocyanine molecule relative to the surface also leads to the lowering of its molecular electronic levels relative to the bands of CdSe. At a tilting angle of 30°, the LUMO level of the dye appears to be lower than the conduction band minimum of cadmium selenide, which makes the electron transfer to its hybridized surface unfavorable. By contrast, the HOMO level of the phenylbutyric acid linker provides a suitable intermediate channel for the hole transfer from the valence band of CdSe to the phthalocyanine that points to the possible acceptor behavior of the phthalocyanine molecule in its hybrids with CdSe nanostructures. acceptedVersion

In this work, we propose new field-free estimators for static field-gradient polarizabilities in finite temperature PIMC simulation. Namely, dipole--quadrupole polarizability $A$, dipole--dipole--quadrupole polarizability $B$ and quadrupole--quadrupole polarizability $C$ are computed for several up to two-electron systems: H, H$^-$, He, Li$^+$, Be$^{2+}$, Ps$_2$, PsH, H$_2^+$, H$_2$, H$_3^+$ and HeH$^+$. We provide complementary data for ground state electronic properties within the adiabatic approximation, and demonstrate good agreement with available values in the literature. More importantly, we present fully non-adiabatic results from 50 K to 1600 K, which allow us to analyze and discuss strong thermal coupling and rovibrational effects in total field-gradient polarizabilities. These phenomena are most relevant but clearly overlooked, e.g., in the construction of modern polarizable force field models. However, our main purpose is demonstrating the accuracy and simplicity of our approach in a problem that is generally challenging., Comment: 6 pages, 2 figures, 2 supplementary pages

Direct sampling of multi-dimensional systems with quantum Monte Carlo methods allows exact account of many-body effects or particle correlations. The most straightforward approach to solve the Schr\"odinger equation, Diffusion Monte Carlo, has been used in several benchmark cases for other methods to pursue. Its robustness is based on direct sampling of a positive probability density for diffusion in imaginary time. It has been argued that the corresponding real time diffusion can not be realised, because the corresponding oscillating complex valued distribution can not be used to drive diffusion. Here, we demonstrate that this can be done with a couple of tricks turning the distribution piecewise positive and normalisable. This study is a proof of concept demonstration using the well-known and transparent case: one-dimensional harmonic oscillator. Furthermore, we show that our novel method can be used to find not only the ground state but also excited states and even the time evolution of a given wave function. Considering fermionic systems, this method may turn out to be feasible for finding the wave function nodes.

The nonlinear optical properties of matter have a broad relevance and many methods have been invented to compute them from first principles. However, the effects of electronic correlation, finite temperature, and breakdown of the Born-Oppenheimer approximation have turned out to be challenging and tedious to model. Here we propose a straightforward approach and derive general field-free polarizability and hyperpolarizability estimators for the path-integral Monte Carlo method. The estimators are applied to small atoms, ions, and molecules with one or two electrons. With the adiabatic, i.e., Born-Oppenheimer, approximation we obtain accurate tensorial ground state polarizabilities, while the nonadiabatic simulation adds in considerable rovibrational effects and thermal coupling. In both cases, the 0 K, or ground-state, limit is in excellent agreement with the literature. Furthermore, we report here the internal dipole moment of PsH molecule, the temperature dependence of the polarizabilities of ${\mathrm{H}}^{\ensuremath{-}}$, and the average dipole polarizabilities and the ground-state hyperpolarizabilities of ${\mathrm{HeH}}^{+}$ and $\mathrm{H}{{}_{3}}^{+}$.

Quantum phase slip (QPS) is the particular manifestation of quantum fluctuations of the order parameter of a current-biased quasi-1D superconductor. The QPS event(s) can be considered a dynamic equivalent of tunneling through conventional Josephson junction containing static in space and time weak link(s). At low temperatures T<, Comment: 3 pages, 3 figures

We present a comparison of the most common finite-size supercell correction schemes for charged defects in density functional theory calculations. Considered schemes include those proposed by Makov and Payne (MP), Lany and Zunger (LZ), and Freysoldt, Neugebauer, and Van de Walle (FNV). The role of the potential alignment is also assessed. Supercells of various sizes are considered and the corrected formation energies are compared to the values obtained by extrapolation to large supercells. For defects with localized charge distributions, we generally find that the FNV scheme slightly improves upon the LZ one, while the MP scheme generally overcorrects except for point-charge-like defects. We also encountered more complex situations in which the extrapolated values do not coincide. Inspection of the defect electronic structure indicates that this occurs when the defect Kohn-Sham states are degenerate with band-edge states of the host. (C) 2011 Elsevier B.V. All rights reserved.

Adsorption of benzene on oxygen rich and reduced SnO 2 surfaces is studied by employing density functional theory calculations, slab model and linear combination of atomic orbitals approach. Rather than preferential adsorption sites, it is found that the adsorption potential energy surface is flat at both surfaces. The bridging oxygen atoms on the stoichiometric surface induce both covalent and ionic bonding leading to weak chemisorptions, whereas bonding on the reduce surface is closer to physisorption. Deformation of the benzene adsorbate due to adsorption is negligible and only small opposite charge transfer is found explaining the differences between the two surfaces.

Siloxane is a favorable candidate as an anchor group that can be used to bind organic molecules to SnO2 surfaces, with a wide range of practical applications. Therefore, adsorption geometries and energies of siloxane coupler on the SnO2 (110) surface have been investigated in this study using quantum-chemical periodic density functional theory (DFT) calculations. We present a comparative study of different siloxane adsorption arrangements on pristine and fluorine doped SnO2 surface. According to the calculations, the surface doping with fluorine leads to stabilization of the siloxane network at the stannic oxide surface. The trend is analyzed in terms of additional charge provided by F impurities to the chemisorbed oxygen atoms thus increasing the ionicity of their bonding. Implications of the current findings for the design of organic-metal oxide interface with better thermo-stability and improved electronic properties are discussed.

An azobenzene molecule Disperse Red 1 with strongly delocalised frontier orbitals has been studied with a number of density-functional theory (DFT) related approaches. The purpose is two-fold: to interpret observed photoabsorption and to compare the performance of various DFT-based approximations. The planarity of the vacuum conformation of the lower energy trans conformation is found to be significantly distorted when transformed to the higher energy cis conformation, and both isomers are found to absorb in the experimentally observed wavelengths in solutions. A common feature in both is essentially forbidden lowest energy absorption because of a negligible overlap of HOMO and LUMO due to symmetry unmatch. We find the time-dependent DFT to be the best approach for quantitative evaluation of photoabsorption energies and intensities, whereas Kohn-Sham eigenenergies and orbitals give good description only qualitatively. Isomerisation and dimerisation energetics have also been evaluated, and trans dimers have found to be up to 1 eV per molecule more stable than single molecules.

Adsorption of three small hydrocarbons molecules (acetylene, ethylene and ethane) on two SnO2(1 1 0) surfaces is studied by employing density functional theory calculations, slab model and linear combination of atomic orbitals (LCAO) approach. Acetylene and ethylene adsorb preferentially on top of a 5-fold tin atom, while ethane aligns itself with the rows of bridging oxygens and settles between two surface tin atoms. Adsorption energies and changes occurring in the structural characteristics and electronic structure of both the hydrocarbons and SnO2 surface indicate chemisorption of unsaturated hydrocarbons and physisorption of ethane on reduced surfaces. In all cases of chemisorption there are two contributing mechanisms: charge transfer creating ionic bonding (down-shift of levels) and covalent bonding (broadening of levels).

Five new pyrrolo[1,2-b]pyridazine derivatives: 5,6-dicarbomethoxy-2,7-dimethylpyrrolo[1,2-b]pyridazine ( I ), 5,6-dicarbomethoxy-7-methyl-2-phenylpyrrolo[1,2-b]pyridazine ( II ), 5,6-dicarbomethoxy-7-ethyl-2-phenylpyrrolo[1,2-b]pyridazine ( III ), 5-carboethoxy-7-methyl-2-phenylpyrrolo[1,2-b]pyridazine ( IV ) and 7-ethyl-2-phenylpyrrolo[1,2-b]pyridazine ( V ), with aryl or methyl substituents in pyridazinic ring and with alkyl, aryl and/or carbometoxy in the pyrrolic ring have been investigated by spectroscopic methods and electronic structure calculations. UV–vis absorption together with steady-state and time-resolved fluorescence measurements have been conducted in cyclohexane, n -hexane (as inert solvents), and in solid state to evidence comparatively the effect of the structure of the compounds on the absorption and fluorescence properties. All five compounds have an intense fluorescence with high quantum yields. The intense fluorescence is evidenced also in solid state. The electronic structure calculations have been performed in the framework of density functional theory (DFT) and time dependent DFT (TD-DFT) in order to elucidate the differences observed in absorption spectra as an effect of the substituents.

The path-integral Monte Carlo simulation method is used to examine one and two electrons in a system of two coupled disc-like quantum dots (QD) in a zero magnetic field. With this approach we are able to evaluate the one-electron distributions and two-electron correlation functions, and finite temperature effects on both. Increase of temperature broadens the distributions as expected, the effect being smaller for correlated electrons than for single ones. The simulated one- and two-particle distributions of a single and two coupled quantum dots are also compared to those from other theoretical methods. For the one-particle distributions we find a good agreement with those from the DFT approach. The effect of the third dimension or the thickness of the almost two-dimensional disc-like QDs is small for the one-particle distributions, but it is clearly seen in the electron-electron correlation or the two-particle distribution function at low temperatures. The mutual Coulomb energy of the two electrons is found to be temperature-independent, and also, independent of the correlation effects on the dynamics. Computational capacity is found to become the limiting factor in simulations with increasing accuracy or increasing number of particles, and in case of fermions in particular. This and other aspects of PIMC and its capability for this type of calculations are also discussed.

Finite temperature quantum behavior of hydrogen adsorbates on Ni(0 0 1) surface is simulated using path-integral Monte Carlo technique. The adsorbate–surface and adsorbate–adsorbate interactions are described by the many-body alloy potential form, fitted to the adsorption parameters from DFT calculations. Temperatures 100 K and 300 K and coverages from 1/8 to 7/8 are considered. Also quantum and classical adsorbate behavior is compared. At low temperatures, the quantum delocalization of the adsorbates is considerable with all studied coverages, and therefore, temperature dependence of distributions is weak. At T = 300 K, however, the H–H interaction energy has a considerable effect on distributions and energetics. By using a semi-classical description of the hydrogen adsorbates both temperature and coverage dependencies become strong at both temperatures.

The path-integral Monte Carlo method is employed to evaluate static (hyper)polarizabilities of small hydrogen systems at finite temperature. Exact quantum statistics are obtained for hydrogen atom and hydrogen molecule immersed in homogeneous electric field. The method proves to be reliable and yields perfect agreement with known values of static polarizabilities in both adiabatic and nonadiabatic simulations. That is, we demonstrate how electronic, rotational, and vibrational contributions can be evaluated either separately or simultaneously. Indeed, at finite temperature and nonzero-field strengths we observe considerable rovibrational effects in the polarization of the hydrogen molecule. Given sufficient computational resources, the path-integral Monte Carlo method turns out to be a straightforward tool for describing and computing static polarizabilities for traditionally challenging regimes.

We observe using ab initio methods that localized surface plasmon resonances in icosahedral silver nanoparticles enter the asymptotic region already between diameters of 1 and 2 nm, converging close to the classical quasistatic limit around 3.4 eV. We base the observation on time-dependent density-functional theory simulations of the icosahedral silver clusters Ag$_{55}$ (1.06 nm), Ag$_{147}$ (1.60 nm), Ag$_{309}$ (2.14 nm), and Ag$_{561}$ (2.68 nm). The simulation method combines the adiabatic GLLB-SC exchange-correlation functional with real time propagation in an atomic orbital basis set using the projector-augmented wave method. The method has been implemented for the electron structure code GPAW within the scope of this work. We obtain good agreement with experimental data and modeled results, including photoemission and plasmon resonance. Moreover, we can extrapolate the ab initio results to the classical quasistatically modeled icosahedral clusters., 8 pages, 5 figures

We have studied the structural and electronic properties of As-rich GaAs1−x Nx and N-rich GaN1−yAsy alloys in a large composition range using first-principles methods. We have systematically investigated the effect of the impurity atom configuration near both GaAs and GaN sides of the concentration range on the total energies, lattice constants and bandgaps. The N (As) atoms, replacing substitutionally As (N) atoms in GaAs (GaN), cause the surrounding Ga atoms to relax inwards (outwards), making the Ga–N (Ga– As) bond length about 15% shorter (longer) than the corresponding Ga–As (Ga–N) bond length in GaAs (GaN). The total energies of the relaxed alloy supercells and the bandgaps experience large fluctuations within different configurations and these fluctuations grow stronger if the impurity concentration is increased. Substituting As atoms with N in GaAs induces modifications near the conduction band minimum, while substituting N atoms with As in GaN modifies the states near the valence band maximum. Both lead to bandgap reduction, which is at first rapid but later slows down. The relative size of the fluctuations is much larger in the case of GaAs1−x Nx alloys. We have also looked into the question of which substitutional site (Ga or N) As occupies in GaN. We find that under Ga-rich conditions arsenic prefers the substitutional N site over the Ga site within a large range of Fermi level values.

A doubly linked porphyrin–fullerene dyad is studied by molecular dynamics (MD) simulations in polar and non-polar solvent, as well as in vacuum. Two conformations representing the most probable vacuum structures are optimised with semi-empirical and density functional methods (DFT) and the structural differences are assessed. The distances between the centres of the porphyrin and fullerene appear very similar in MD simulations regardless of the environment. Optimization with DFT calculations produces centre-to-centre distances that are within 0.07–5% of the corresponding MD values of realistic environments. The results validate the electronic structure calculations relevant to describing photoinduced electron-transfer and related properties in the dyad.

Surface properties of solids and the interactions between molecules and solid surfaces are important for many technical applications. They also involve a range of physical and chemical phenomena of fundamental scientific interest. The importance of oxygen chemistry at SnO 2 surfaces follows from the fact that SnO 2 is used as an active material in gas sensor applications. The operation principle of these sensors is usually based on measurable conductance response of the material, which is understood in terms of reactions of gas molecules with different oxygen species adsorbed onto the surface. The role of the lattice oxygen, but in particular, the bridging oxygen atoms on SnO 2 surfaces, is also active. Detailed understanding of the reaction mechanisms of various oxygen species at SnO 2 surfaces is important, as it offers a way to improve the sensitivity and selectivity of the sensors. Oxygen adsorption–desorption kinetics at the SnO 2 surface is studied experimentally using O 2 -temperature-programmed desorption (TPD) method together with conductance measurements in the case of SnO 2 powder and polycrystalline thick films made from the powder. In addition, CO-TPD is studied and the transient behaviour of various oxygen species is considered. Molecular beam epitaxy (MBE) was also used to fabricate polycrystalline and monocrystalline thin films with the SnO 2 (101) face on single crystal sapphire substrate. Simultaneous surface potential and conductance measurements during heating and cooling in different ambient atmospheres were used to characterize the monocrystalline SnO 2 (101) surface after various surface treatments.

The electronic structure of GaAs(1 1 0) surface is analyzed using Density Functional Theory (DFT-GGA) in atomic orbital basis (LCAO). The surface orbitals and the corresponding local density of electronic states (LDOS) are calculated for purposes of interpreting STM images. We show how local atomic orbitals of surface atoms are related to tunneling channels for electrons in STM imaging. A destructive interference between orbitals of two neighbouring atoms increases the contrast between the two atoms, and this is reflected in directionality of STM patterns of GaAs(1 1 0) surfaces. We also discuss how the basic formalism of Tersoff–Hamann approach to STM simulation can be reformulated to reveal the role of phase difference between tunneling channels.

Surface structures of In2O3 films have been studied by employing experimental methods for surface characterization together with theoretical modeling for geometries, electronic structures and surface energetics. Polycrystalline In2O3 films deposited by spray pyrolysis were analyzed by using XRD, HRTEM and XPS methods. It is found that the most abundant (4 0 0) surface face reconstructs considerably leading to formation of surface mono-oxygen and di-oxygen forms. This uppermost surface layer exhibit high ability to reduction/oxidation processes during the thermal treatment. The unsaturated indium ions appearing at the reconstructed surface serve as the active sites for the chemisorbed oxygen species. Therefore, the mechanisms of In2O3 sensitivity to reducing gases include both “redox” and catalytic effects in a very thin surface layer. On the other hand, response of In2O3-based gas sensors to oxidizing gases is limited by diffusion type processes.

Features of SnO2(1 1 0) surfaces with a surplus electron are considered computationally by applying density functional method. The additional charge is found to influence on surface energetics and surface relaxations considerably with respect to the corresponding surfaces without additional charge. This effect is pronounced in the cases of oxidized surfaces, where different oxygen adsorbates (O, O2 and O3) tend to capture the charge. On stoichiometric surface, these oxygen species are adsorbed on-top of the five-fold coordinated surface tin atoms and are attributed to adsorbed O−, O 2 - and O 3 - species associated with spin polarization. Based on evaluated total energy differences formation of these species are compared. In cases of stoichiometric and slightly oxygen deficient surfaces without adsorbates, charge accumulation is qualitatively similar to the case of bulk SnO2 with a considerable amount of the surplus charge enriched around Sn atoms. A pronounced charge accumulation is observed around under-coordinated surface tin atoms of stoichiometric and oxygen deficient (1 1 0) surfaces.

In the present work density functional theory (DFT) is applied to calculate geometries and electronic structures of 3,4-dimethyl- N -{2-[10,15,20-tris-(3,5-di- tert -butyl-phenyl)-porphyrin-5-yl]-phenyl}benzamide (H 2 P–O34) and its zinc complex (ZnP–O34). The influences of the substituents in the porphyrin ring on the energies of frontier molecular orbitals are evaluated. This is done by gradually increasing the amount of substituents in the investigated porphine and zinc porphine rings. The DFT framework used in this work is found to correctly describe the differences in electronic structures, which arise from differences in the location, number, and electronic effects of the substituents at the meso -positions of the porphyrin ring. In addition, the applicability of the ground state DFT method for evaluating the one-electron transition energies, which correspond to the near UV–Vis optical absorptions of porphyrin molecules, is investigated. The transition energies and the orbitals involved in electronic transitions are well reproduced by this method, as shown by comparing the results to our time dependent density functional calculations.

Sol–gel adhesive material for isostatic hotpressing lamination process was synthesized using 3-aminopropyltrimethoxysilane (APTES), 3-glycidyloxypropyltrimethoxysilane (GPTS) and methacryloxypropyltrimethoxysilane (MPTS) as precursors. Reaction dynamics between APTES and GPTS was followed on-line with Raman spectroscopy. The lamination process was optimized by varying hotpressing times and pressures at the constant temperature. Mechanical shear strength properties were found to be excellent for studied composition, 25–30 kg/cm2 at its best. One possible application of laminated polycarbonate (PC) structure is a soldier helmet visor. Therefore, the ballistic protection properties were tested by shooting with 1.1 gram standard fragment, and determining the v50 velocity value, which corresponds to 50% perforation. The laminated structure gave 5.7% better protection compared with the solid polycarbonate. The laminated components were found to have high optical transparency at the visible wavelengths as well as high environmental stability.

The path-integral Monte Carlo method is used to examine the two-electron state of a model quantum dot. Electrons in the two-dimensional quantum dot are confined by a harmonic oscillator potential of strength ω=1eV. Mixed state densities, energies and pair correlation functions are evaluated at various temperatures, and their temperature dependencies are analyzed. Also, the two-electron pure state energetics is resolved and the correlation induced shifts of the first and second excited states are evaluated.

Structural aspects of crystalline tin oxide and its interfaces with composition Sn2O3 are considered computationally based on first principles density functional calculations. The possibility of formation of different nonstoichiometric tin oxide crystals and SnO2/SnO interfaces is shown. The lowest total energy per Sn2O3 unit was evaluated for a layered Sn2O3 crystal, where oxygen vacancies are arranged into the (101) plane in a rutile structure system. Interface structures with orientations SnO2(101)/SnO(001) and SnO2(100)/SnO(100), corresponding to composition Sn2O3 are only slightly less stable. Their estimated interface energies are 0.15 J m−2 and 0.8 J m−2, respectively. All geometries have components similar to well-known rutile structure SnO2 and litharge structure SnO geometries. Most stable Sn2O3 crystals include SnO6 octahedra similar to those found in rutile structure SnO2.

Palladium adatoms adsorbed on various SnO 2 (1 1 0) surfaces with sub-monolayer coverages is studied theoretically by using the density functional method. At low coverages, the binding is found to occur essentially between Pd, surface Sn atoms. Twofold coordinated bridging oxygens of stoichiometric (1 1 0), (1 0 0), (1 0 1)SnO 2 surfaces also bind to Pd, but the threefold coordinated in-plane oxygens are inert. At low Pd coverages, the stability of Pd adsorbates is slightly increased due to interaction between Pd atoms which occupy neighboring SnO 2 (1 1 0) surface unit cells. This can lead to alignment of Pd adatoms on the (1 1 0) surface along [0 0 1] direction. Among the different sites that Pd may occupy and be adsorbed at, the in-plane oxygen vacancies at reduced SnO 2 (1 1 0) are predicted to be preferred. Substitution of tin atoms by palladium is energetically most favorable in the cases of least coordinated surface tin atoms of oxygen deficient surface. Main features of the evaluated electronic structures and charge distributions of the Pd/SnO 2 (1 1 0) surfaces are considered.