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Charge traps in the semiconductor bulk (bulk charge traps) make it difficult to predict the electric field within wide-bandgap semiconductors. The issue is the daunting number of bulk charge-trap candidates which means the treatment of bulk charge traps is generally qualitative or uses generalized models that do not consider the trap's particular electronic structure. The electric field within a wide-bandgap semiconductor is nonetheless a crucial quantity in determining the operation of semiconductor devices and the performance of solid-state single-photon emitters embedded within the semiconductor devices. In this work we accurately compute the average electric field measured at the location of N$V^-$ charged defects for the substitutional N (N$_\text{C}$) concentration of $n_{\text{N}_\text{C}} \approx 1.41\times10^{18}$ cm$^{-3}$ for the commonly used oxygen-terminated diamond (see [D. A. Broadway $et$ $al$., Nature Electronics 1, 502 (2018)]). We achieve this result by evaluating the leading-order contribution to the electric field far away from the surface, which comes from the N$_\text{C}$ defects that induce the ionization of the N$V^-$. Our results use density-functional theory (DFT) and the principle of band bending. Our work has the potential to aid both in the prediction of the functioning of semiconductor devices and in the prediction and correction of the spectral diffusion that often plagues the optical frequencies of solid-state single-photon emitters upon repeated photoexcitation measurements. Our results for the timescales involved in thermally driven charge transfer also have the potential to aid in investigations of charge dynamics.

The hyperfine interaction is an important probe for understanding the structure and symmetry of defects in a semiconductor. Density functional theory has shown that it can provide useful first-principles predictions for both the hyperfine tensor and the hyperfine constants that arise from it. Recently there has been great interest in using group-IV impurity-vacancy color centers $\mathrm{X}{V}^{\ensuremath{-}}$ (where X = Si, Ge, Sn, or Pb and $V$ is a carbon vacancy) for important applications in quantum computing and quantum information science. In this paper we have calculated the hyperfine tensors for these $\mathrm{X}{V}^{\ensuremath{-}}$ color centers using the HSE06 screened Hartree-Fock hybrid exchange-correlation functional with the inclusion of core electron spin polarization. We have compared our results to calculations which only use the PBE exchange-correlation functional without the inclusion of core electron spin polarization and we have found that our results are in very good agreement with available experimental results. Finally, we have theoretically shown that these $\mathrm{X}{V}^{\ensuremath{-}}$ color centers exhibit a Jahn-Teller distortion which explains the observed anisotropic distribution of the hyperfine constants among the neighboring $^{13}\mathrm{C}$ nuclear spins.

The nitrogen-vacancy (N V) lattice defect in diamond, consisting of an N substitutional atom and an adjacent C vacancy, is commonly observed in two charge states, negative (N V−) and neutral (N V0). The N V− defect exhibits spin state-dependent fluorescence and is, therefore, amenable to optical methods for spin-state readout, while the N V0 is not. Hence, the N V− defect is much more useful for quantum sensing and quantum information processing. However, only N V0 electroluminescence has been observed, even from centers showing N V− in photoluminescence. In the present work, we use first-principles electronic structure calculations to determine adiabatic charge transition levels for the conversion of N V− to N V0 in the presence of substitutional N or P impurities, which provide the charge of the N V− center. We find that the adiabatic charge transition levels in the presence of such impurities lie at energies close to or lower than the zero-phonon line of the N V− center and that these energies only decrease as the concentration of N donors increases. This work, therefore, elucidates the absence of observed electroluminescence from the N V− and proposes a path toward observation of the phenomenon.

The vibrational properties of octahedrane (C

Recently there has been much interest in using Group IV elements from the Periodic Table to fabricate and study X$V$ color centers in diamond where X = Si, Ge, Sn, or Pb and $V$ is a carbon vacancy. These Group IV color centers have a number of interesting spin and optical properties which could potentially make them better candidates than N$V^-$ centers for important applications in quantum computing and quantum information processing. Unfortunately, the very same ion implantation process that is required to create these X$V$ color centers in diamond necessarily also produces many carbon vacancies ($V_{\rm C}$) which can form complexes with these color centers ($V_{\rm C}-$X$V$) that can dramatically affect the properties of the isolated X$V$ color centers. The main focus of this work is to use density-functional theory (DFT) to study the thermodynamics and kinetics of the formation of these $V_{\rm C}-$X$V$ complexes and to suggest experimental ways to impede this process such as varying the Fermi level of the host diamond material through chemical doping or applying an external electrical bias. We also include a discussion of how the simple presence of many $V_{\rm C}$ can negatively impact the spin coherence times ($T_2$) of Group IV color centers through the presence of acoustic phonons., 19 pages, 7 figures

The vibrational properties of octahedrane (C12H12) are calculated using density-functional theory employing two different computational methods: an all-electron Gaussian orbital approach and a Naval Research Laboratory-tight-binding scheme (NRL-TB) coupled with molecular dynamics (NRL-TBMD). Both approaches yield vibrational densities of states for octahedrane that are in good general agreement with each other. NRL Molecular Orbital Library can also provide accurate infrared and Raman spectra which can be analyzed and compared with experimental results, while NRL-TBMD can be conveniently scaled up for larger finite-temperature simulations. This latter approach is used in our paper to produce a theoretical prediction for a stable room temperature structure of octahedrane.

Objectives The purpose of this study was to investigate issues related to access to endodontic care in North Carolina for individuals who used dental public health resources such as public health clinics (PHCs) or private practices that accept Medicaid or other government-sponsored reimbursement programs private practices that accept Medicaid (PPM). Methods Surveys were sent to 1,195 dentists regarding frequency and type of endodontic conditions encountered, treatments provided, and perceived barriers to care. Results were analyzed using logistic regression with the level of significance set at 0.05. Results Five hundred forty-six surveys were returned for a 45.7% response rate. Of the respondents, 79% reported frequently encountering an endodontic condition, but only 34% reported performing any type of definitive endodontic procedure. Graduates after the year 2000 were significantly more likely to perform definitive endodontic procedures (P

Diamondoids are rigid stable hydrocarbons of unusually high symmetry. The recent isolation of many diamondoids of varying shapes and sizes from petroleum oil has revitalized interest in this field. We show in this work that modern density-functional theory (DFT) methods are capable of predicting the structural, electronic, and vibrational properties of a specific diamondoid molecule, cyclohexamantane (C 26 H 30 ), and that our calculated properties are in good agreement with experimental results. Our calculations also demonstrate that DFT can be an important tool in characterizing the properties of even larger diamondoids of interest to experimentalists.

Decamethyldizincocene (Zn2(η5-Cp*)2) is the first example of a stable organometallic compound with a homonuclear metal Zn–Zn bond. Although its structural properties have been characterized by X-ray diffraction and high-resolution mass spectroscopy, it has been suggested that its experimental infrared and Raman spectra might not be particularly useful in identifying the Zn–Zn stretch mode in Zn2(η5-Cp*)2. We have used density-functional theory (DFT) to calculate both an infrared (IR) and Raman spectra for Zn2(η5-Cp*)2. In particular, our results clearly identify a number of low frequency Raman active modes wherein Zn–Zn twist and stretch modes are clearly identifiable.

The lower-order diamondoid hydrocarbon molecule, cyclohexamantane (C 26 H 30 ), has been recently isolated from distilled Gulf Coast petroleum. While the structure of C 26 H 30 has been confirmed through X-ray diffraction, mass spectroscopy, and 1 H/ 13 C NMR spectroscopy, its vibrational Raman spectra has only been identified through an indirect comparison with the experimental Raman spectra for adamantane and diamond. We used density-functional theory (DFT) to calculate a Raman spectra whose frequencies and relative intensities are in excellent agreement with the experimental Raman spectra for C 26 H 30 , thus providing direct vibrational proof of its existence.

In this Letter we report a density functional all-electron calculation of the structural and electronic properties of the polynitrogen high-energy molecule, azidopentazole (N8). We have also performed a vibrational analysis to determine the IR and Raman spectra. Our calculated geometrical properties and vibrational frequencies are in good agreement with previous ab initio and density functional calculations. The weaker IR modes show a different relative ordering than previously reported. We also report calculated Raman intensities for azidopentazole.

We have performed density functional calculations on the recently discovered energetic molecule, octanitrocubane C 8 (NO 2 ) 8 . We predict a strongly exothermic dissociation energy and calculate the ionization potentials, electron affinities and vibrational spectra. We predict that the isolated molecule exhibits a symmetry which is higher than that found in the solid. The vibrational density of states shows strong Raman activity at low frequencies and strong IR intensities at high frequencies. We identify a low-energy anharmonic mode which is consistent with the experimentally assumed free torsional rotations of the NO 2 groups about their respective C–N axes.

The reaction of HOCl + HCl → Cl2 + H2O in the presence of chlorine anion Cl− has been studied using ab initio methods. The overall exothermicity is 15.5 kcal mol−1 and this reaction has been shown to have a high activation barrier of 46.5 kcal mol−1. Cl− is found to catalyze the reaction via the formation of HOCl·Cl−, ClH·HOCl·Cl− and Cl−·H2) intermediate ion-molecule complexes or by interacting with a concerted four-center transition state of the reaction of HOCl + HCl.

The purpose of this study was to investigate issues related to access to endodontic care in North Carolina for individuals who used dental public health resources such as public health clinics (PHCs) or private practices that accept Medicaid or other government-sponsored reimbursement programs private practices that accept Medicaid (PPM).Surveys were sent to 1,195 dentists regarding frequency and type of endodontic conditions encountered, treatments provided, and perceived barriers to care. Results were analyzed using logistic regression with the level of significance set at 0.05.Five hundred forty-six surveys were returned for a 45.7% response rate. Of the respondents, 79% reported frequently encountering an endodontic condition, but only 34% reported performing any type of definitive endodontic procedure. Graduates after the year 2000 were significantly more likely to perform definitive endodontic procedures (P 0.05). Lack of insurance was the greatest barrier to care with 89% considering it a moderate to major barrier, followed by cost of the endodontic treatment (87%) and cost of the restoration following treatment (86%). PPMs were more likely to consider cost and insurance a major barrier (P 0.05).In North Carolina public health and Medicaid settings, the frequency of endodontic treatments provided was much lower than the frequency of endodontic conditions encountered that might have benefited from treatment. Graduation year was the best indicator for the provision of root canal therapy. Additionally, treatment patterns and perceptions of barriers to care are different for PHCs and PPMs.

Ab initio calculations are used to investigate the energetics of the reaction of C2(X 1Σ+ g) with CH4. The heat of reaction is predicted to be exothermic by 5·4 kcal mol-1 and the barrier height is 0·2 kcal mol-1 at the QCISD(T)/6-311++G(2df,2p)//UMP2/6-311G(2df,2p) level of theory. The implications of the results are discussed in terms of their importance to the atmospheric evolution of ethane oceans on Titan.

Resonant tunneling of electrons is studied in a new class of multi-barrier semiconductor heterostructures which contain graded quantum wells , whose slopes can be changed by varying the alloy composition of the constituent material. Such systems, in the presence of an externally applied bias, can exhibit resonant tunneling behavior that is quite different from previously studied multi-barrier materials, because of the unique coupling of the quasi-bound states between the graded quantum wells. In this paper, we explicitly compute the electron transmission coefficient T ( E ) for such systems as a function of the multi-barrier geometry and internal slopes of the quantum wells to obtain novel transmission coefficient spectra and discuss their implications.

The electronic structure, vibrational spectra, and thermodynamic stability of CCCN and HCCCN have been determined using ab initio molecular orbital theory. The heat of formation for HCCCN was determined using Gaussian‐2 theory (G2) and was found to be 91.5±2 kcal mol−1. There is good agreement between the G2 value and that determined using an isodesmic reaction (i.e., one in which both the number and types of bonds are conserved), which yields 90.7±2 kcal mol−1. The heat of formation for the CCCN radical is 178.2±2 kcal mol−1. These data are used to compute the bond dissociation energies in HCCCN, D0(C–C) and D0(C–H), which are important in assessing the chemistry in Titan’s atmosphere, as well as in other interstellar media.

We solve the one-dimensional Schr\"odinger wave equation in the effective-mass approximation for the transmission coefficient T(E) of electrons through asymmetric multibarrier semiconductor heterostructures in the presence of a constant applied electric field, using an exact Airy-function formalism and the transfer-matrix technique. In particular, we show that for appropriate choices of asymmetry in the barrier widths and heights of the semiconductor heterostructure, the transmission coefficient is enhanced to yield resonances that are stronger than those calculated in symmetric structures, thus giving further validity to Mendez's concept of effective-barrier symmetry for obtaining optimal resonant tunneling in asymmetric double- and triple-barrier semiconductor heterostructures. These results should assist experimental efforts in designing resonant-tunneling systems that require optimum peaks both in the transmission spectrum and current density.

Peer review organizations (PRO) review the health care given to Medicare beneficiaries. If a PRO determines that a hospital admission was unnecessary, the hospital is not reimbursed for the medical care and the case is "denied." The average hospital denial rate on Ohio is 2%; however, the denial rate for hospital admissions of one day or less is 7.5%. An analysis of hospitals and diagnoses for one-day hospital admission denials was performed, showing that urban hospitals and disorders of the circulatory and digestive systems predominate. The inappropriate utilization of health care resources costs millions of dollars per year in Ohio and could be costing the nation billions of dollars. There is an urgent need to research why resources are used less effectively by some hospitals and for some diseases.

In this paper, we report {ital ab initio} calculation of the structural and electronic properties of solid cubane (s-C{sub 8}H{sub 8}) in the local-density approximation. By using an {ital ab initio} constant pressure extended molecular dynamics method with variable cell shape proposed by Wentzcovitch, Martins, and Price, we compute a lattice parameter {ital a} and a bond angle {alpha} for the rhombohedral Bravais lattice and compare it with experimental x-ray data. We obtain bond lengths for the mononuclear C{sub 8}H{sub 8} unit of basis atoms, as well as a density of states and heat of formation. {copyright} {ital 1998} {ital The American Physical Society}

The recent synthesis of a four-fold silylated sila-adamantane molecule (C(24)H(72)Si(14), T(d)), [J. Fischer, J. Baumgartner, and C. Marschner, Science 310, (2005) 825] is the first attempt of making the silicon analogue of adamantane. It has adamantane-like Si(10) core, capped by methyl and silymethyl ligands. We report its electronic structure, vibrational spectrum, and the infra-red and Raman spectra calculated within the density functional formalism using large polarized Gaussian basis sets. The properties of sila-adamantane are compared with exact silicon analogue (Si(10)H(14)) of adamantane. Results show that replacing hydrogens in Si(10)H(16) by methyl and silymethyl ligands results in expansion of the Si(10) core and are responsible for large number of peaks in the Raman spectrum. The Si-C stretch at 664 cm(-1) and methyl deformations frequencies compare well with recent measurements of vibrational frequencies of methylated silicon surface.

Recently an inorganic fullerine-like [As@Ni(12)@As(20)](3-) onion with near-perfect icosahedral symmetry in the crystalline phase was reported [M. J. Moses, J. C. Fettinger, and B. W. Eichhorn, Science 300, 778 (2003)]. This paper presents a detailed computational study in the framework of density functional theory on various aspects of this molecule. The electronic structure of the As@Ni(12)@As(20) is investigated in its neutral as well as -3 charged state together with its subunits As(20) and As@Ni(12) by the all electron linear combination of Gaussian-type orbitals method. The bonding is studied by examining the integrated charge within atomic sphere, the electron localization function, changes in the electron density distribution, and from vibrational modes. We find that strong covalent As-As bonds seen in isolated As(20) become weaker in the As@Ni(12)@As(20) and strong covalent As-Ni bonds are formed. The structural stability of all four clusters is examined by analyzing the energetics and by calculating the vibrational frequencies. Further, the infrared and Raman spectra is predicted for both the neutral and charged As@Ni(12)@As(20) clusters. Finally, the energy barrier for removal of a single arsenic atom is calculated for the neutral As@Ni(12)@As(20) cluster.

It has been shown within density-functional theory that in Mn$_{12}$-acetate there are effects due to disorder by solvent molecules and a coupling between vibrational and electronic degrees of freedom. We calculate the in-plane principal axes of the second-order anisotropy caused by the second effect and compare them with those of the fourth-order anisotropy due to the first effect. We find that the two types of the principal axes are not commensurate with each other, which results in a complete quenching of the tunnel-splitting oscillation as a function of an applied transverse field., Will be presented at MMM conference 2004

We present the ab initiostudy of the geometry, electronic structure, charged states, bonding, and vibrational modes of the recently synthesized fullerene-like $\mathrm{As}@{\mathrm{Ni}}_{12}@{\mathrm{As}}_{20}$ cluster which has icosahedral point symmetry [M. J. Moses, J. C. Fettinger, and B. W. Eichhorn, Science, 300, 778 (2003)]. We show that the molecule is vibrationally stable and will be electronically most stable in its $\ensuremath{-}3$ oxidation state in the condensed phase and in $\ensuremath{-}2$ state in the gas phase. We examine the bonding in this unusually structured molecule from charge transfer between atoms, infrared and Raman spectra, and charge-density isosurfaces.

The dimeric form of the single-molecule magnet $[{\mathrm{Mn}}_{4}{\mathrm{O}}_{3}{\mathrm{Cl}}_{4}({\mathrm{O}}_{2}\mathrm{CEt}{)}_{3}(\mathrm{py}{)}_{3}{]}_{2}$ recently revealed interesting phenomena: no quantum tunneling at zero field and tunneling before magnetic-field reversal. This is attributed to substantial antiferromagnetic exchange interaction between different monomers. The intermolecular exchange interaction, electronic structure, and magnetic properties of this molecular magnet are calculated using density-functional theory within generalized-gradient approximation. Calculations are in good agreement with experiment.

The assessment of pollutant effects on health status requires the mergence and analysis of two different databases: pollution measurements and health care information. This paper compares two subsets of these data: Ohio Environmental Protection Agency data on ambient air pollutants and Ohio Medicare data on respiratory diseases. Small area analysis was performed to assess statewide variations in hospital admission rates for respiratory diseases. The ambient air pollutant levels for each small area were compared to the variations in respiratory disease rates. Five groups of diseases correlated with pollutant levels. In addition, pollutant levels were significantly associated with medical complications. This study demonstrates the feasibility and benefit of linking environmental and health care databases and suggests the need for a more comprehensive, automated analysis of more pollutants and diseases.

Recent advances in molecular beam epitaxy have renewed research on the physics of artificially structured magnetic superlattices. In particular, there has been much theoretical research on the propagation of magnetic spin waves or magnetic polaritons in magnetic superlattices.1 In this work, we have studied the effect of modulating both the period of an antiferromagnetic/non-Magnetic semi-infinite superlattice and the relative thickness of its individual layers to see how the dispersion relationships co (k) for bulk and surface magnetic polaritons are effected. We have also calculated the effect of an external magnetic field on (u (k) and our calculation goes beyond the magnetostatic approximation by taking retardation effects into account.

As an initial step toward the synthesis and characterization of sila-diamondoids, such as sila-adamantane (Si(10)H(16),T(d)), the synthesis of a fourfold silylated sila-adamantane molecule (C(24)H(72)Si(14),T(d)) has been reported in literature [Fischer et al., Science 310, 825 (2005)]. We present the electronic structure, ionization energies, quasiparticle gap, and the excitation energies for the Si(14)(CH(3))(24) and the exact silicon analog of adamantane Si(10)H(16) obtained at the all-electron level using the delta-self-consistent-field and transitional state methods within two different density functional models: (i) Perdew-Burke-Ernzerhof generalized gradient approximation and (ii) fully analytic density functional (ADFT) implementation with atom dependent potential. The ADFT is designed so that molecules separate into atoms having exact atomic energies. The calculations within the two models agree well, to within 0.25 eV for optical excitations. The effect of structural relaxation in the presence of electron-hole-pair excitations is examined to obtain its contribution to the luminescence Stokes shift. The spin-influence on exciton energies is also determined. Our calculations indicate overall decrease in the absorption, emission, quasiparticle, and highest occupied molecular orbital-lowest unoccupied molecular orbital gaps, ionization energies, Stokes shift, and exciton binding energy when passivating hydrogens in the Si(10)H(16) are replaced with electron donating groups such as methyl (Me) and trimehylsilyl (-Si(Me)(3)).

An analysis of Ohio's Medicare data base by the state's Peer Review Organization, using the most common diagnosis-related group in the Medicare population (heart failure and shock), from January 1, 1989 to January 1, 1991, identified 72 cases with confirmed quality-of-care problems. The analysis was performed to determine whether the majority of quality-of-care problems are related to systems or performance deficiencies. Study results indicated that health care workers are being inappropriately blamed for problems that are inherent in the health care system--74% of problems were related to inefficiencies in the health care delivery system, and 26% were determined to reflect performance problems.

Adjusted admission rates for respiratory distress (COPD, asthma, bronchitis, and pneumonia) varied up to 3.09-fold between the highest and lowest hos pital market areas in 1986 for the state of Ohio. Reasons for the variability can be determined through small area analysis techniques with the help of area physicians. Substantial improvements in the availability, delivery, and cost of respiratory care would reasonably be anticipated as a result of such analysis and feedback.

Recently an inorganic fullerine-like [As@Ni12@As20]3− onion with near-perfect icosahedral symmetry in the crystalline phase was reported [M. J. Moses, J. C. Fettinger, and B. W. Eichhorn, Science 300, 778 (2003)]. This paper presents a detailed computational study in the framework of density functional theory on various aspects of this molecule. The electronic structure of the As@Ni12@As20 is investigated in its neutral as well as −3 charged state together with its subunits As20 and As@Ni12 by the all electron linear combination of Gaussian-type orbitals method. The bonding is studied by examining the integrated charge within atomic sphere, the electron localization function, changes in the electron density distribution, and from vibrational modes. We find that strong covalent As-As bonds seen in isolated As20 become weaker in the As@Ni12@As20 and strong covalent As-Ni bonds are formed. The structural stability of all four clusters is examined by analyzing the energetics and by calculating the vibrational...

The CC bond dissociation energy (Do) in cyanogen is predicted using many‐body perturba‐ tion (MP) and quadratic configuration interaction theory [QCISD(T)]. The dissociation limit is poorly predicted with many‐body perturbation theory. At the QCISD(T)/6‐311G(3df)//QCISD(T)/ 6‐311G(2d) level of theory, the bond dissociation energy is predicted to be 132.8±2 kcal mol−1, which is in excellent agreement with experiment.

We show that our exact one‐dimensional Airy function formalism for studying electron resonant tunneling in multibarrier semiconductor heterostructures is an improvement on a previous calculation of Brennan and Summers [J. Appl. Phys. 61, 614 (1987)]. We also clearly demonstrate that our method gives better agreement with the numerical approach of Vassell, Lee, and Lockwood [J. Appl. Phys. 54, 5206 (1983)] in calculating the transmission coefficient T(E) and current density J(E) for multibarrier semiconductor heterostructures.

The empirical pseudopotential method (EPM) is used to calculate electronic charge densities at selected k points of the valence and conduction band edges of two II–VI semiconductors: ZnSe and CdTe.

The empirical pseudopotential method is used to compute electronic charge densities at the \ensuremath{\Gamma}, L, and X k points of the conduction-band edge in selected group-IV elemental (Si, Ge, \ensuremath{\alpha}-Sn) and III-V compound (GaP, GaAs, GaSb, InP, InAs, InSb) semiconductors. We find that these charge distributions are essentially independent of the system under study and are strongly dependent upon the symmetry of the wave function of a particular state. As a direct consequence of this analysis, we find that the ${\ensuremath{\Gamma}}_{1}^{c}$ conduction-band state for the heteropolar semiconductors contains an abundance of charge density associated with the anion as opposed to the cation in support of a previous observation by Baldereschi and Maschke, and that the symmetry of the wave function of the conduction-band edge at ${X}^{c}$ for GaSb is actually ${X}_{3}^{c}$, and not ${X}_{1}^{c}$ as has been previously assigned in the literature. We therefore predict an increase in both the direct and indirect band gaps of the Nowotny-Juza alloy \ensuremath{\alpha}-LiZnSb upon applying the interstitial insertion rule of Wood, Zunger, and de Groot to GaSb.

We present a pseudopotential local-density calculation for Be/sub 2/ using a molecular supercell scheme. The results for the binding energy (-0.43 eV), equilibrium bond length (4.53 a.u.), and ground-state vibrational frequency (373 cm/sup -1/) are in reasonable agreement with recent measurements and other quantum-chemical calculations.

La methode de pseudopotentiel empirique, qui utilise une base d'ondes planes etendue, ainsi que la methode LCAO, qui utilise une base d'orbitales gaussiennes «on-site», sont capables de calculer des densites de charge en des points determines de la zone de Brillouin de semi-conducteurs. On le montre en calculant la structure de bande et les densites de charge de GaAs par les deux methodes