Search results for "Quantum"
showing 10 items of 9714 documents
InBO3 and ScBO3 at high pressures: an ab initio study of elastic and thermodynamic properties
2016
We have theoretically investigated the elastic properties of calcite-type orthoborates ABO(3) (A= Sc and In) at high pressure by means of ab initio total-energy calculations. From the elastic stiffness coefficients, we have obtained the elastic moduli (B, G and E), Poisson's ratio (nu), B/G ratio, universal elastic anisotropy index (A(U)), Vickers hardness, and sound wave velocities for both orthoborates. Our simulations show that both borates are more resistive to volume compression than to shear deformation (B > G). Both compounds are ductile and become more ductile, with an increasing elastic anisotropy, as pressure increases. We have also calculated some thermodynamic properties, like D…
The Coordination of Uranyl in Water: A Combined Quantum Chemical and Molecular Simulation Study
2005
The coordination environment of uranyl in water has been studied using a combined quantum mechanical and molecular dynamics approach. Multiconfigurational wave function calculations have been performed to generate pair potentials between uranyl and water. The quantum chemically determined energies have been used to fit parameters in a polarizable force field with an added charge transfer term. Molecular dynamics simulations have been performed for the uranyl ion and up to 400 water molecules. The results show a uranyl ion with five water molecules coordinated in the equatorial plane. The U-O(H(2)O) distance is 2.40 A, which is close to the experimental estimates. A second coordination shell…
Crystal structure of sinhalite MgAlBO4 under high pressure
2015
We report on high-pressure angle-dispersive X-ray diffraction data up to 27 GPa for natural MgAlBO4 sinhalite mineral and ab initio total energy calculations. The experimental bulk modulus of sinhalite is B-0 = 171(3) GPa with a first-pressure derivative of B-0' = 4.2(3). A comparison with other olivine-type compounds shows that the value for B0 is 27% larger than that of Mg2SiO4 forsterite and 29% smaller than that of Al2BeO4 chrysoberyl. These differences are interpreted, on the basis of our ab initio calculations, in terms of the relative incompressibility of Al-O bonds in AlO6 octahedra (with a calculated bulk modulus of 250(1) GPa) as compared to Mg-O bonds in MgO6 octahedra (with a ca…
Abelian integrals and limit cycles
2006
Abstract The paper deals with generic perturbations from a Hamiltonian planar vector field and more precisely with the number and bifurcation pattern of the limit cycles. In this paper we show that near a 2-saddle cycle, the number of limit cycles produced in unfoldings with one unbroken connection, can exceed the number of zeros of the related Abelian integral, even if the latter represents a stable elementary catastrophe. We however also show that in general, finite codimension of the Abelian integral leads to a finite upper bound on the local cyclicity. In the treatment, we introduce the notion of simple asymptotic scale deformation.
Alien limit cycles near a Hamiltonian 2-saddle cycle
2005
Abstract It is known that perturbations from a Hamiltonian 2-saddle cycle Γ can produce limit cycles that are not covered by the Abelian integral, even when it is generic. These limit cycles are called alien limit cycles. This phenomenon cannot appear in the case that Γ is a periodic orbit, a non-degenerate singularity, or a saddle loop. In this Note, we present a way to study this phenomenon in a particular unfolding of a Hamiltonian 2-saddle cycle, keeping one connection unbroken at the bifurcation. To cite this article: M. Caubergh et al., C. R. Acad. Sci. Paris, Ser. I 340 (2005).
ON THE CALCULATION OF THE HEAT CAPACITY IN PATH INTEGRAL MONTE CARLO SIMULATIONS
1992
In Path Integral Monte Carlo simulations the systems partition function is mapped to an equivalent classical one at the expense of a temperature-dependent Hamiltonian with an additional imaginary time dimension. As a consequence the standard relation linking the heat capacity Cv to the energy fluctuations, <E2>−<E>2, which is useful in standard classical problems with temperature-independent Hamiltonian, becomes invalid. Instead, it gets replaced by the general relation [Formula: see text] for the intensive heat capacity estimator; β being the inverse temperature and the subscript P indicates the P-fold discretization in the imaginary time direction. This heatcapacity estimator…
The vibrational levels of methane obtained from analyses of high-resolution spectra
2006
International audience; Methane and its tetrahedral isotopologues are spherical-top molecules whose high-resolution rovibrational spectra can only be analyzed in detail, thanks to sophisticated symmetry-adapted tensorial models. However, the effective Hamiltonian parameters of such models do not give direct access to the positions of the vibrational sublevels. In this paper, we present a calculation of the vibrational level positions for 12CH4, 13CH4, 12CD4 and 13CD4 performed using the effective Hamiltonian parameters obtained through recent analyses. We also include the results of a re-analysis of the octad system of 12CH4 performed with a higher order of the development which slightly im…
Spectroscopic and quantum chemical study of pressure effects on solvated chlorophyll
2004
Abstract Hydrostatic pressure effects up to 907 MPa on absorption spectra of chlorophyll a molecules dissolved in diethyl ether have been studied at ambient temperature both experimentally and theoretically using quantum chemistry methods. The fluorescence spectra are studied only experimentally. The data suggest that coordination interactions between the central Mg atom of the chlorophyll and solvent molecules along with interactions that modify the porphyrin skeleton of the solute are responsible for the observed differences of pressure dependence of the Q y , Q x , and Soret spectral bands. The coordination number of the Mg atom changes from five to six between 400 and 600 MPa.
Biexciton formation and exciton coherent coupling in layered GaSe.
2015
Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarizati…
Theoretical Study of the Electronic Excited States of Tetracyanoethylene and Its Radical Anion
2005
The low-lying electronic states of tetracyanoethylene (TCNE) and its radical anion were studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The results obtained yield a full interpretation of the electronic absorption spectra, explain the spectral changes undergone upon reduction, give support to the occurrence of a bound excited state for the anionic species, and provide valuable information for the rationalization of the experimental data obtained with electron transmission spectroscopy.