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The absorption spectra of congenetic wurtzite (WZ) and zincblende (ZB) CdS magic-sized clusters are investigated. We demonstrate that the exciton peak positions can be tuned by up to 500 meV by varying the strong coupling between X-type ligands and the semiconductor cores, while the addition of L-type ligands primarily affects cluster midgap states. When Z-type ligands are displaced by L-type ligands, red shifts in the absorption spectra are observed, despite the fact there is a small decrease in cluster size. Density functional theory calculations are used to explain these findings and they reveal the importance of Cd and S dangling bonds on the midgap states during the Z- to L-type ligand exchange process. Overall, ZB CdS clusters show higher chemical stability than WZ clusters but their optical properties exhibit greater sensitivity to the solvent. Conversely, WZ CdS clusters are not stable in a Lewis base-rich environment, resulting in various changes in their spectra. Our findings enable researchers to select capping ligands that modulate the optical properties of semiconductor clusters while maintaining precise control over their solvent interactions. [ABSTRACT FROM AUTHOR]

The positively charged nitrogen vacancy (NV +) center in diamond has been traditionally treated as a dark state due to the experimental lack of an optical signature. Recent computational studies have shown that it is possible for the NV + defect to have an excited state transition equivalent to that of the negatively charged (NV −) center, but no photoluminescence (PL) predictions have been reported so far. We report the first ab initio calculation showing that the NV + center presents quantum emission, with zero phonon line at 765 nm and a non-zero transition dipole moment, approximately one quarter of the transition dipole moment of NV −. We calculate the energy levels of the multielectron states under the time-dependent density functional theory (singlet and triplet E states), and using our recently developed frequency cutoff method, we predict the full PL spectrum. Our results suggest that this state cannot be considered intrinsically "dark" and charge specific quenching mechanisms should be investigated as the cause of the lack of optical activity in experimental characterizations. [ABSTRACT FROM AUTHOR]

This work presents algorithms for the efficient enumeration of configuration spaces following Boltzmann-like statistics, with example applications to the calculation of non-radiative rates, and an open-source implementation. Configuration spaces are found in several areas of physics, particularly wherever there are energy levels that possess variable occupations. In bosonic systems, where there are no upper limits on the occupation of each level, enumeration of all possible configurations is an exceptionally hard problem. We look at the case where the levels need to be filled to satisfy an energy criterion, for example, a target excitation energy, which is a type of knapsack problem as found in combinatorics. We present analyses of the density of configuration spaces in arbitrary dimensions and how particular forms of kernel can be used to envelope the important regions. In this way, we arrive at three new algorithms for enumeration of such spaces that are several orders of magnitude more efficient than the naive brute force approach. Finally, we show how these can be applied to the particular case of internal conversion rates in a selection of molecules and discuss how a stochastic approach can, in principle, reduce the computational complexity to polynomial time. [ABSTRACT FROM AUTHOR]

Measuring the photoluminescence of defects in crystals is a common experimental technique for analysis and identification. However, current theoretical simulations typically require the simulation of a large number of atoms to eliminate finite-size effects, which discourages computationally expensive excited state methods. We show how to extract the room-temperature photoluminescence spectra of defect centers in bulk from an ab initio simulation of a defect in small clusters. The finite-size effect of small clusters manifests as strong coupling to low frequency vibrational modes. We find that removing vibrations below a cutoff frequency determined by constrained optimization returns the main features of the solid-state photoluminescence spectrum. This strategy is illustrated for the negatively charged nitrogen vacancy defect in diamond (NV −) presenting a connection between defects in solid state and clusters; the first vibrationally resolved ab initio photoluminescence spectrum of an NV − defect in a nanodiamond; and an alternative technique for simulating photoluminescence for solid-state defects utilizing more accurate excited state methods. [ABSTRACT FROM AUTHOR]

Geometries and electronic structures of germanium trimer clusters doped with titanium TiGe3−/0 were studied making use of the complete active space self-consistent field followed by second-order perturbation theory explicitly correlated coupled cluster singles and doubles method with perturbative triples corrections CCSD(T)-F12 and Tao-Perdew-Staroverov-Scuseria methods. Two electronic states (²A′ and ²A″) of the anion (pyramid shape) were determined to be nearly degenerate and energetically competing for the anionic ground state of TiGe3−. These two anionic states are believed to be concurrently populated in the experiment and induce six observed anion photoelectron bands. Total 14 electronic transitions starting from the ²A′ and ²A″ states were assigned to five out of six visible bands in the experimental anion photoelectron spectrum of TiGe3−. Each band was proven to be caused by multiple one-electron detachments from two populated anionic states. The last experimental band with the highest detachment energy is believed to be the result of various inner one-electron removals. [ABSTRACT FROM AUTHOR]

Perylene diimide (PDI) derivatives are widely used materials for luminescent solar concentrator (LSC) applications due to their attractive optical and electronic properties. In this work, we study aggregation-induced exciton quenching pathways in four PDI derivatives with increasing steric bulk, which were previously synthesized. We combine molecular dynamics and quantum chemical methods to simulate the aggregation behavior of chromophores at low concentration and compute their excited state properties. We found that PDIs with small steric bulk are prone to aggregate in a solid state matrix, while those with large steric volume displayed greater tendencies to isolate themselves. We find that for the aggregation class of PDI dimers, the optically accessible excitations are in close energetic proximity to triplet charge transfer (CT) states, thus facilitating inter-system crossing and reducing overall LSC performance. While direct singlet fission pathways appear endothermic, evidence is found for the facilitation of a singlet fission pathway via intermediate CT states. Conversely, the insulation class of PDI does not suffer from aggregation-induced photoluminescence quenching at the concentrations studied here and therefore display high photon output. These findings should aid in the choice of PDI derivatives for various solar applications and suggest further avenues for functionalization and study. [ABSTRACT FROM AUTHOR]

In this paper we discuss a new simulation method that can be used to predict preferred surface reconstructions of model systems by Molecular Dynamics (MD). The method overcomes the limitations imposed by periodic boundary conditions for finite boundary MD simulations which can normally prevent reconstructions. By simulating only the reconstructed surface layer and by removing the periodic boundary effects and the free energy barriers to reconstruction, the method allows surfaces to reconstruct to a preferred structure. We test the method on three types of surfaces: (i) the Au(100) and Pt(100) hexagonally reconstructed surface, (ii) the Au(111) herringbone surfaces, and (iii) the triangularly reconstructed Ag surface layer on a Pt(111) substrate and find the method readily finds lower surface energy reconstructions as preferred by the potential. [ABSTRACT FROM AUTHOR]

We derive new quantum Monte Carlo (QMC) estimators for the electronic density at the position of a point nucleus using the zero-variance and zero-bias principles. The resulting estimators are highly efficient, and are significantly simpler to implement and use than alternative methods, as they contain no adjustable parameters. In addition, they can be used in both variational and diffusion QMC calculations. Our best estimator is used to calculate the most accurate available estimates of the total electron density at the nucleus for the first-row atoms Li-Ne, the Ar atom, and the diatomic molecules B2, N2, and F2. [ABSTRACT FROM AUTHOR]

We derive efficient quantum Monte Carlo estimators for the anisotropic intracule and extracule densities. These estimators are used in conjunction with an accurate explicitly correlated wave function to investigate the bond-length dependence of electron correlation effects in the ground-state H2 molecule. It is shown that the localized increase in the magnitude of the correlation energy as the bond is stretched is accompanied by highly anisotropic correlation effects. In addition, we find a small long-range part of the Coulomb hole, which is present even at the equilibrium bond length. [ABSTRACT FROM AUTHOR]

An accurate description of electron correlation is essential for the calculation of interaction energies in cases where dispersion energy is a major component, for example, for the rare gas atoms, physisorption on graphite, and graphene-graphene interactions. Such calculations are computationally demanding using supermolecule methods and the energies calculated lack a simple, physical interpretation. Alternatively density functional theories (DFTs) may be used to give an approximate estimate of the correlation energy. However, the physical nature of this DFT estimate of electron correlation energy is not well understood and, in fact, most current DFT methods do not describe dispersion energy at all. Hence, an analysis of the correlation energy contribution to interaction energies where dispersion energy is important is needed. In order to do this we provide an analysis of the correlation energy contribution to the potential energy curves of He2, Ne2, and Ar2 in terms of the Hartree–Fock (HF) interaction term ΔEintHF, a dispersion energy term Edisp and an electron correlation term ΔEintC. ΔEintC includes all other correlation energy effects besides Edisp and is shown to be repulsive, of a similar short range character to, but of smaller magnitude than ΔEintHF. This analysis was used to develop a theoretical model which gives a very good estimate of the potential energy wells for He2, Ne2, Ar2, HeNe, HeAr, and NeAr. [ABSTRACT FROM AUTHOR]

We have simulated the vacuum deposition and subsequent growth of gold nanoparticles on various substrates in order to explore the effects that substrate morphology has on the resultant morphology of gold nanoparticles. The substrates and conditions explored included, the three low index faces, namely, (111), (100), and (110) for both fcc and bcc crystalline substrate structures, including various substrate lattice constants and temperatures. Firstly, we cataloged the major nanoparticle morphologies produced overall. While some substrates were found to produce a mixture of the main nanoparticle morphologies we were successful in identifying certain substrates and temperature conditions for which only Ih, Dh, or certain fcc crystalline nanoparticles can be grown almost exclusively. The substrate characteristics, temperature conditions, and governing growth dynamics are analyzed. We shed light on the balance between substrate influences and vacuum growth tendencies. From observations we can speculate that a substrate alters both the free energy stability of gold nanoparticles and/or the free energy barriers to transformation between certain morphologies. As such we find that substrates are an effective tool in templating the selective growth of desired nanoparticles or surface nanostructures. [ABSTRACT FROM AUTHOR]

A simple method is presented which ensures the electron-nucleus cusp condition is satisfied by the Slater-Jastrow wavefunctions commonly employed in quantum Monte Carlo simulations. The method is applied in variational energy calculations of the neon atom and a selection of molecules using both Gaussian and Slater basis sets. In addition, we discuss the relationship between the electron-nucleus cusps and the variance of forces, and investigate the sensitivity of forces to the quality of the cusps for various diatomic molecules. [ABSTRACT FROM AUTHOR]

We have studied the thermodynamic and kinetic growth mechanisms behind the formation of the “pancake” decahedron (Dh) gold nanoparticle using computer simulation. Free energy calculations showed that the full pancake morphology is thermodynamically unstable across all the nanoparticle size ranges studied. However, from observations of growth simulations we discovered that a kinetic transport mechanism plays a significant contributing role in the formation process through a transfer of adatoms from the top and bottom (111) Dh faces to the side (100) faces. More specifically we observed how diffusing adatoms on the (111) face are at times “pulled” off this face and into the (111)-(100) edge of the Dh, forcing a row of (100) side atoms into a (1×5) hexagonal reconstruction. Subsequently, this row of atoms was observed to buckle and then deconstruct forcing adatoms out onto the (100) side face completing the transfer. This transport mechanism is shown to be the main kinetic driving force behind the growth of the thermodynamically unstable pancake Dh nanoparticle. The observed mechanism has implications for the nonequilibrium morphologies of nanoparticles involving a (100)-(111) surface boundary, especially for systems with surface reconstructions which increase the density of the surface. [ABSTRACT FROM AUTHOR]

We calculate and compare the relative free energies of ideal/pristine gold nanoparticles for morphologies produced previously in vapor synthesis computer simulations. The results in conjunction with previous work provide a unique and direct quantitative comparison between ideal thermodynamics and kinetics in the synthesis of gold nanoparticles for an identical system. The ideal/pristine free energies suggest that the Ih morphology was the most stable structure up to the 147(Ih) followed by the TOh for all the remaining nanoparticle sizes. A grouping of m-Dh structures was identified in the size range N=146–318 with stabilities which were very close to the most stable Ih and TOh structures. The free energy analysis was somewhat at odds with population statistics obtained from our kinetic growth simulations where the Ih dominated and where very little presumably stable TOh nanoparticles were produced, implying that kinetic mechanisms are more influential than thermodynamic considerations. On the other hand other possible reasons for such discrepancies are discussed; one of these includes an interesting observation where the Ih morphology was found to have a unique ability to incorporate exposed surface disorder such as adatoms into stable hexagonal surface structures through internal and surface structural rearrangements, leading to a possible enhancement in stabilities of Ih-type morphologies. [ABSTRACT FROM AUTHOR]

In this computational study, we used molecular dynamics and the embedded atom method to successfully reproduce the growth of gold nanorod morphologies from starting spherical seeds in the presence of model surfactants. The surfactant model was developed through extensive systematic attempts aimed at inducing nonisotropic nanoparticle growth in strictly isotropic computational growth environments. The aim of this study was to identify key properties of the surfactants which were most important for the successful anisotropic growth of nanorods. The observed surface and collective dynamics of surfactants shed light on the likely growth phenomena of real nanoprods. These phenomena include the initial thermodynamically driven selective adsorption, segregation, and orientation of the surfactant groups on specific crystallographic surfaces of spherical nanoparticle seeds and the kinetic elongation of unstable surfaces due to growth inhibiting surfactants on those surfaces. Interestingly, the model not only reproduced the growth of nearly all known nanorod morphologies when starting from an initial fcc or fivefold seed but also reproduced the experimentally observed failure of nanorod growth when starting from spherical nanoparticles such as the Ih morphology or morphologies containing a single twinning plane. Nanorod morphologies observed in this work included fivefold nanorods, fcc crystalline nanorods in the [100] direction and [112] directions and the more exotic “dumbell-like” nanorods. Non-nanorod morphologies observed included the Ih and the nanoprism morphology. Some of the key properties of the most successful surfactants seemed to be suggestive of the important but little understood role played by silver ions in the growth process of real nanorods. [ABSTRACT FROM AUTHOR]

The authors use a newly fitted gold embedded atom method potential to simulate the initial nucleation, coalescence, and kinetic growth process of vapor synthesized gold nanoparticles. Overall the population statistics obtained in this work seemed to mirror closely recent experimental HREM observations by Koga and Sugawara [Surf. Sci. 529, 23 (2003)] of inert gas synthesized nanoparticles, in the types of nanoparticles produced and qualitatively in their observance ratio. Our results strongly indicated that early stage coalescence (sintering) events and lower temperatures are the mainly responsible for the occurrence of the Dh and fcc based morphologies, while “ideal” atom by atom growth conditions produced the Ih morphology almost exclusively. These results provide a possible explanation as to why the Dh to Ih occurrence ratio increases as a function of nanoparticle size as observed by Koga and Sugawara. [ABSTRACT FROM AUTHOR]

In this study we use a new topological structure measure to analyze the local environment of 923 atom gold clusters quenched from the melt, at various quench rates, by molecular dynamics. The crystallization and geometrical rearrangements of the core atoms upon freezing can be clearly observed using our structure measure which is based on planar graphs. Our results support the hypothesis that crystallization is initiated from the surface and proceeds into the cluster core. [ABSTRACT FROM AUTHOR]

We fit a new gold embedded atom method (EAM) potential using an improved force matching methodology which included fitting to high-temperature solid lattice constants and liquid densities. The new potential shows a good overall improvement in agreement to the experimental lattice constants, elastic constants, stacking fault energy, radial distribution function, and fcc/hcp/bcc lattice energy differences over previous potentials by Foiles, Baskes, and Daw (FBD) [Phys. Rev. B 33, 7983 (1986)] Johnson [Phys. Rev. B 37, 3924 (1988)], and the glue model potential by Ercolessi et al. [Philos. Mag. A 50, 213 (1988)]. Surface energy was improved slightly as compared to potentials by FBD and Johnson but as a result vacancy formation energy is slightly inferior as compared to the same potentials. The results obtained here for gold suggest for other metal species that further overall improvements in potentials may still be possible within the EAM framework with an improved fitting methodology. On the other hand, we also explore the limitations of the EAM framework by attempting a brute force fit to all properties exactly which was found to be unsuccessful. The main conflict in such a brute force fit was between the surface energy and the liquid lattice constant where both could not be fitted identically. By intentionally using a very large number of spline sections for the pair potential, electron-density function, and embedding energy function, we eliminated a lack of functional freedom as a possible cause of this conflict and hence can conclude that it must result from a fundamental limitation in the EAM framework. [ABSTRACT FROM AUTHOR]

Previously we developed a general method for calculating the free energy of any surface constrained to a distinct surface excess number/density. In this paper we show how to combine a range of such surfaces, whose free energies have been calculated, to produce an ad hoc semigrand canonical ensemble of surfaces from which ensemble surface properties can be calculated, including the ensemble surface free energy. We construct such an ensemble for the disordered Au(100) semihexagonal reconstructed surface using a Glue model potential at 1000 K and calculate the ensemble surface free energy to be 0.088 18 eV/Å2. The ensemble average surface lateral density was found to be 1.375 (with respect to the bulk), which is in agreement with previous grand canonical Monte Carlo studies. [ABSTRACT FROM AUTHOR]

Recently a method termed constrained fluid λ-integration was proposed for calculating the free energy difference between bulk solid and liquid reference states via the construction of a reversible thermodynamic integration path; coupling the two states in question. The present work shows how the application of the constrained fluid λ-integration concept to solid/liquid slab simulation cells makes possible a generally applicable computer simulation methodology for calculating the free energy of any surface and/or surface defect structure, including surfaces requiring variations in surface atom or density number, such as the (1×5) Au(100) or (1×2) missing row Au(110) reconstructed surfaces or excess adatom/vacancy/step populated surfaces. We evaluate the methodology by calculating the free energy of various disordered high temperature Au(110) embedded atom method surfaces constrained to differing excess surface atom numbers [including those corresponding to the (1×2) missing row reconstructed surface] and obtained the interesting result that at 1000 K (as distinct from lower temperatures) the free energy difference between these surfaces is reduced to zero; a result which is consistent with an expected order-disorder phase transition for the Au(110) surface at such high temperatures. [ABSTRACT FROM AUTHOR]

The cell parameters, bulk moduli and electronic densities-of-states (DOS) of pure and vacancy defect AlN were computed using generalized-gradient approximation (GGA) and hybrid functional (B3LYP) computational methods within both plane wave-pseudopotential and localized Gaussian basis set approaches. All of the methods studied yielded cell parameters and bulk moduli in reasonable agreement with experiment. The B3LYP functional was also found to predict an optical band gap in excellent agreement with experiment. These methods were subsequently applied to the calculation of the geometry, defect state positions and formation energies of the cation (V[sub Al]) and anion (V[sub N]) single vacancy defects. For the V[sub Al] defect, the plane wave-pseudopotential predicted a significant retraction of the neighboring N away from the vacancy, while for the V[sub N] defect, only slight relaxations of the surrounding Al atoms towards the vacancy were predicted. For the computed DOS of both vacancy defects, the GGA methods yielded similar features and defect level positions relative to the valence band maximum, while the B3LYP method predicted higher separations between the defect levels and the valence and conduction bands, leading to higher energy occupied defect levels. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Previously a new universal λ-integration path and associated methodology was developed for the calculation of “exact” surface and interfacial free energies of solids. Such a method is in principle applicable to any intermolecular potential function, including those based on ab initio methods, but in previous work the method was only tested using a relatively simple embedded atom method iron potential. In this present work we apply the new methodology to the more sophisticated and more accurate modified embedded atom method (MEAM) iron potential, where application of other free- energy methods would be extremely difficult due to the complex many-body nature of the potential. We demonstrate that the new technique simplifies the process of obtaining “exact” surface free energies by calculating the complete set of these properties for the low index surface faces of bcc and fcc solid iron structures. By combining these data with further calculations of liquid surface tensions we obtain the first complete set of exact surface free energies for the solid and liquid phases of a realistic MEAM model system. We compare these predictions to various experimental and theoretical results. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have used the density functional theory (DFT) method with a plane wave-pseudopotential basis to compute the structure and properties of a model xanthate molecule (HOCS[sup -, sub 2]) and its adsorption characteristics on the pyrite FeS[sub 2](110) and (111) surfaces. Molecular calculations revealed that HOCS[sup -, sub 2] and CH[sub 3]CH[sub 2]OCS[sup -, sub 2] have similar head group electronic structure and, therefore, the use of the model xanthate is a justifiable approximation in simulations of xanthate head group attachment to FeS[sub 2] surfaces. Results from DFT calculations suggest that HOCS[sup -, sub 2] readily undergoes dissociation at the fourfold coordinated surface Fe on the (110) surface, and the bridging S of the (111) surface. These results suggest that xanthate may undergo chemisorption at defect sites on real FeS[sub 2] surfaces, which contain low-coordinate Fe sites and sites in proximity to cleaved S-S bonds. [ABSTRACT FROM AUTHOR]

Previously we studied a range of λ-integration paths, specifically designed for calculating surface and interfacial free energies of solids with disordered surfaces or interfaces, using molecular dynamics or Monte Carlo simulation methods. Some of these were successfully applied to the stable low index (100) and (110) Fe bcc surfaces, up to temperatures high enough (1200 K) to induce the onset of surface disorder via the formation of adatoms. Here we apply these same methods to the high energy (111) bcc Fe face, where the "ideal" surface structure was found to be metastable at low temperatures. The results showed that application of paths used in our previous study lead to irreversibility. Hence we further refine the paths with the development of a much more powerful and general path, which we termed the "blanket lambda" path. We show the newest path to be reversible and to provide "exact" surface free energy reference points for the stable and metastable surface structures of the (111) bcc Fe face. We also show this general path to be applicable to the low index (100) and (110) faces. [ABSTRACT FROM AUTHOR]

Previously we studied λ-integration paths for the calculation of "exact" surface and interfacial free energies that were limited to simulation methods where atomic interactions can be scaled using a multiplicative parameter λ, as is the case for analytical empirical potentials. Here we develop new reversible paths and associated λ-integration methodology to find exact surface and interfacial free energies of solids, that could potentially be used in conjunction with any intermolecular potential function and/or methods such as ab initio simulations, where one cannot trivially scale and sum the slab interactions as is done with "simple" classical intermolecular potentials. As a first step we test our paths and methodology on the (100), (110), and (111) faces of a-iron using embedded atom method interactions. We find accurate agreement with our previous surface free energy calculations for all faces, including the highly disordered (111) face. [ABSTRACT FROM AUTHOR]

We study λ-integration paths, specifically designed for calculating "exact" surface and interfacial free energies of solids at elevated temperatures using molecular dynamics or Monte Carlo simulation methods. We compare various paths with the standard technique of thermodynamic integration by application to the surface free energy for the (100) and (110) faces of alpha iron using embedded atom method (EAM) potentials. We demonstrate which paths are completely reversible at high temperatures and show consistency of results for these paths. The λ-integration paths can be applied with confidence to find equilibrium surface free energies, within the limits of the surface area, intermolecular potentials and other approximations implicit to the simulation methods used. [ABSTRACT FROM AUTHOR]

In this study we use a new topological structure measure to analyze the local environment of 923 atom gold clusters quenched from the melt, at various quench rates, by molecular dynamics. The crystallization and geometrical rearrangements of the core atoms upon freezing can be clearly observed using our structure measure which is based on planar graphs. Our results support the hypothesis that crystallization is initiated from the surface and proceeds into the cluster core.

Recently a method termed constrained fluid lambda-integration was proposed for calculating the free energy difference between bulk solid and liquid reference states via the construction of a reversible thermodynamic integration path; coupling the two states in question. The present work shows how the application of the constrained fluid lambda-integration concept to solid/liquid slab simulation cells makes possible a generally applicable computer simulation methodology for calculating the free energy of any surface and/or surface defect structure, including surfaces requiring variations in surface atom or density number, such as the (1 x 5) Au(100) or (1 x 2) missing row Au(110) reconstructed surfaces or excess adatom/vacancy/step populated surfaces. We evaluate the methodology by calculating the free energy of various disordered high temperature Au(110) embedded atom method surfaces constrained to differing excess surface atom numbers [including those corresponding to the (1 x 2) missing row reconstructed surface] and obtained the interesting result that at 1000 K (as distinct from lower temperatures) the free energy difference between these surfaces is reduced to zero; a result which is consistent with an expected order-disorder phase transition for the Au(110) surface at such high temperatures.