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  • 1
    Electronic Resource
    Electronic Resource
    Molecular dynamics study of the structural and thermodynamic properties of MgO crystal with quantum correction (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 489-494 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have applied the constant temperature and pressure molecular dynamics technique to the simulation of structural and thermodynamic properties of the MgO crystal in the temperature range between 300 and 2000 K, using empirical pair potential functions consisted of the Coulomb, dispersion, and repulsion interactions. Quantum corrections are included, based on the Wigner–Kirkwood expansion of the free energy in powers of Planck constant h. In the present study, only the first nonvanishing term with the order h2 is considered. As expected, quantum effects on the properties are found to become increasingly important with decreasing temperature. In view of the simplicity of the potential used, our simulation is quite successful in reproducing a wide range of measured properties of MgO, including the crystal structure, bulk modulus, thermal expansion coefficient, heat capacity, and mean-square atomic displacements over a wide temperature range above 500 K. However, at lower temperatures, both the heat capacity and thermal expansion coefficient of MgO are computed to decrease too sharply with decreasing temperature when compared with experiment. This indicates the necessity of including higher-order quantum correction terms at low temperatures for a more accurate description.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457484
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  • 2
    Electronic Resource
    Electronic Resource
    Breathing shell model in molecular dynamics simulation: Application to MgO and CaO (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 3304-3309 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics (MD) simulation is used to calculate the elastic constants of both MgO and CaO at zero pressure, and their temperature dependences, as well as the temperature–pressure–volume equation of states of the two oxides. The interionic potential is taken to be the sum of pairwise additive Coulomb, van der Waals, and repulsive interactions. In order to account for the observed large departures from the Cauchy relation of the elastic constants of the two oxides, the breathing shell model (BSM) is introduced in MD simulation, in which the repulsive radii of O ions are allowed to deform isotropically under the effects of other ions in the crystal, with each core and breathing shell being linked by a harmonic spring with force constant k. Required energy parameters, including k, were derived empirically to reproduce the observed molar volumes and elastic constants of the two oxides at ambient conditions, and their temperature dependences as accurately as possible. The MD simulation with BSM is very satisfactory in reproducing accurately not only the observed large Cauchy violations,but also the measured molar volumes and individual elastic constants of the two oxides over wide temperature ranges, at zero pressure. In addition the MD simulation reproduces accurately the measured volume compression data of CaO up to 60 GPa at 300 K. Here we present the MD simulated temperature–pressure–volume equation of state of CaO as a useful internal pressure calibration standard at high temperatures and high pressures. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475727
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  • 3
    Electronic Resource
    Electronic Resource
    Simulation of the pre-melting behaviour of MgSiO 3 perovskite at high pressures and temperatures (1991)
    [s.l.] : Nature Publishing Group
    Nature 351 (1991), S. 735-737 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] There is controversy concerning the electrical conductivity of (Mg, Fe)SiO3 perovskite under lower-mantle temperatures and pressures. Li and Jeanloz1'2 have measured the electrical con-ductivity of (Mg, Fe)SiO3 perovskite at typical lower-mantle temperatures and pressures, and concluded that (Mg, ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/351735a0
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  • 4
    Electronic Resource
    Electronic Resource
    Limited regions of the α2-domain α-helix control anti-A2 allorecognition: an analysis using a panel of A2 mutants (1991)
    Springer
    Immunogenetics 34 (1991), S. 149-156 
    Details
    ISSN: 1432-1211
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract The regions of the HLA-A2 molecule controlling anti-A2 alloreactivity were explored using naturally occurring allelic variants of HLA-A, and a panel of transfectants expressing the products of A2.1 genes that had been mutated at multiple positions encoding residues in the α2 domain α-helix. As a means of detecting distant conformational effects, these altered A2.1 molecules were also examined serologically. Amino acid substitutions at the carboxy-terminal end of the α2 domain α-helix led to diminished staining with the monoclonal antibody (mAb) MA2.1. The epitope for this antibody has previously been mapped to the α1 domain α-helix (residues 62–65). This suggests that interdomain contacts may cause conformational alteration, and that mutants can have distant, as well as local effects. Of the 24 positions where substitutions were made, only six led to loss of the anti-A2 alloresponse by the three clones and three lines that were tested. In addition, the mutations that altered the MA2.1 epitope, located on the α1 domain α-helix, did not inhibit allorecognition. This suggests that a limited number of regions on the A2.1 molecule are responsible for allodeterminant expression. The most influential substitutions were those at positions 152, 154, 162, and 166. It is notable that three of these are predicted to be T-cell receptor (Tcr)-contacting residues, and one (152) to contribute to peptide binding. These results suggest that the specificity of alloreactive T cells is determined by exposed polymorphisms, directly contacted by the Tcr, and by concealed polymorphisms which influence peptide binding.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00205817
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  • 5
    Electronic Resource
    Electronic Resource
    A cluster of mutations in HLA-A2 α2 helix abolishes peptide recognition by T cells (1991)
    Springer
    Immunogenetics 34 (1991), S. 141-148 
    Details
    ISSN: 1432-1211
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract In order to investigate the regions of HLA-A2 that control peptide-specific cytotoxic T lymphocyte (CTL) recognition, 37 HLA-A2 genes coding for 50 point mutations that span the α2 helix were synthesized by the technique of saturation mutagenesis. Twenty-nine of these genes, which code for 41 point mutations, were transfected into C1R cells and used as targets in cytotoxicity assays, in the presence of influenza-A matrix peptide 58–68 with specific CTL as effectors. All the transfectants were recognized fully by matrix peptide-specific CTL apart from those with amino acid substitutions at positions 152, 154, 155, 156, or 161, which led to a total loss of recognition and those with mutations at residue 27 or a double mutation at 138 and 150, which were recognized in an intermediate manner. The clustering of the crucial residues that emerges may reflect direct interaction of their side-chains with peptide or the CTL receptor.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00205816
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  • 6
    Electronic Resource
    Electronic Resource
    Computer simulation of the MgSiO3 polymorphs (1992)
    Springer
    Physics and chemistry of minerals 18 (1992), S. 365-372 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Six polymorphs of MgSiO3 have been studied using molecular dynamic (MD) simulation techniques, based on the empirical potential (MAMOK), which is composed of terms to describe pairwise additive Coulomb, van der Waals attraction, and repulsive interactions. Crystal structures, bulk moduli, volume thermal expansivities, and enthalpies were simulated for the known MgSiO3 polymorphs; orthoenstatite, clinoenstatite, protoenstatite, garnet, ilmenite, and perovskite. The simulated values compare very well with the available experimental data, and the results are quite satisfactory in view of the diversity of the crystal structures of the six polymorphs, the wide range of simulated properties, and the simplicity of the MAMOK potential. MD simulation was further successfully used to study the possibile existence of a post-protoenstatite phase at high temperature, and a C2/c phase at high pressure, both phases being suggested or inferred previously from experimental works.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00199417
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  • 7
    Electronic Resource
    Electronic Resource
    Computational model of the structural and elastic properties of the ilmenite and perovskite phases of MgSiO3 (1987)
    Springer
    Physics and chemistry of minerals 14 (1987), S. 101-106 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract The structural and elastic properties of the ilmenite and perovskite phases of MgSiO3 are investigated with a computational model based on energy minimization. The potential energies of these two crystals are approximated by the sum of Coulomb, van der Waals, and repulsion terms between atoms. Required energy parameters are derived by fitting the parameters to the observed crystal structures of these two phases as well as to the measured elastic constants of the ilmenite phase. The resulting potential model is applied to predicting the elastic constants of the perovskite phase. The calculated bulk modulus of the perovskite phase compares favorably with the data obtained from volume-compression experiments as well as the values estimated from empirical elasticity systematics of perovskite type compounds. The predicted shear modulus of the perovskite phase is also in reasonable agreement with the values proposed from similar empirical elasticity systematics. Subsequently, the model is used to simulate the high pressure behaviors of the crystal structures and elastic constants of these two phases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00308213
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  • 8
    Electronic Resource
    Electronic Resource
    Molecular dynamics study of MgSiO3 perovskite (1988)
    Springer
    Physics and chemistry of minerals 16 (1988), S. 234-238 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract The molecular dynamics (MD) method is applied to the simulations of the structural and physical properties of MgSiO3 perovskite. The potential energy of our model consists of pairwise additive Coulomb and repulsive terms between atoms, and van der Waals attraction terms for oxygen-oxygen interactions. Required energy parameters are transferred from empirical potentials previously developed for MgSiO3 perovskite and MgSiO3 ilmenite based on a static lattice simulation technique. Minor modifications are made for the repulsive radii in the potential to account for thermal vibrational effects in the static description of a crystal lattice. In spite of the simplicity of the potentials used, the MD simulations have succeeded in reproducing pretty well a wide range of structural and physical properties of MgSiO3 perovskite, including the crystal structure, the compressibilities, the thermal expansivities, and the mean-square atomic displacements. The MD method was further applied to predict the high temperature and high pressure behavior of MgSiO3 perovskite.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00220690
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  • 9
    Electronic Resource
    Electronic Resource
    Molecular dynamics study of the structures and bulk moduli of crystals in the system CaO-MgO-Al2O3-SiO2 (1996)
    Springer
    Physics and chemistry of minerals 23 (1996), S. 345-353 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Molecular dynamics (MD) simulations have been used to calculate the structures and bulk moduli of crystals in the system CaO-MgO-Al2O3-SiO2 (CMAS) using an interatomic potential model (CMAS94), which is composed of pairwise additive Coulomb, van der Waals, and repulsive interactions. The crystals studied, total of 27, include oxides, Mg meta- and ortho-silicates, Al garnets, and various Ca or Al bearing silicates, with the coordination number of cations ranging 6 to 12 for Ca, 4 to 12 for Mg, 4 to 6 for Al, and 4 and 6 for Si. In spite of the simplicity of the CMAS94 potential and the diversity of the structural types treated, MD simulations are quite satisfactory in reproducing well the observed structural data, including the crystal symmetries, lattice parameters, and average and individual nearest neighbour Ca-O, Mg-O, Al-O, and Si-O distances. In addition MD simulated bulk moduli of crystals in the CMAS system compare well with the observed values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00199500
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  • 10
    Electronic Resource
    Electronic Resource
    High-temperature Raman spectroscopy and quasi-harmonic lattice dynamic simulation of diopside (1997)
    Springer
    Physics and chemistry of minerals 24 (1997), S. 440-446 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract  We investigated the lattice vibrational properties and lattice dynamical behaviour of diopside by combining laser micro-Raman spectroscopic measurements with quasi-harmonic lattice dynamic simulation using a transferable interatomic potential. We obtained polarized Raman spectra from a Fe-poor natural diopside and the temperature dependencies of the Raman modes to 1125 K from high-temperature Raman spectra of a Fe-poor and a Fe-rich natural diopside. The various modes display different temperature dependencies: from −0.021 cm−1/K to −0.004 cm−1/K. The temperature shift of low frequency modes is generally higher. A comparison of experimentally determined frequencies and symmetries of vibrational modes of the optical type (Raman and infrared) obtained in this and earlier studies with those calculated by us suggests that a consistent characterization of the vibrational properties was achieved. The good agreement between the experimental and simulated data on the temperature-dpendencies of the Raman modes (within 5%), crystal structure (2%), bulk modulus (5%), volume thermal expansivity (6%), and constant volume heat capacity (0.2%) testifies to the applicability of the transferable interatomic potential and the lattice dynamic model to predicting the vibrational, physical, and thermodynamic properties. The simulated properties from the lattice dynamic calculations are very similar to those obtained by molecular dynamic calculations with the same potential model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002690050058
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