Recent Publications

Jorge Kohanoff

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  1. Title: General local and rectilinear vibrational coordinates consistent with Eckart’s conditions

    Author(s): Scivetti I., Kohanoff J., Gidopoulos N.,

    Physical Review A, 79, No. 3, Art. No. 032516 (MAR 2009)

    doi: 10.1103/PhysRevA.79.032516

    We present a general method to construct a set of local rectilinear vibrational coordinates for a nonlinear molecule whose reference structure does not necessarily correspond to a stationary point of the potential-energy surface. We show both analytically and with a numerical example that the vibrational coordinates satisfy Eckart’s conditions. In addition, we find that the Watson Hamiltonian provides a fairly robust description even of highly excited vibrational states of triatomic molecules, except for a few states of large amplitude motion sampling the singular region of the Hamiltonian. These states can be identified through slow convergence.

  2. Title: Self-consistent geometry in the computation of the vibrational spectra of molecules

    Author(s): Scivetti I., Kohanoff J., Gidopoulos N.,

    Physical Review A, 80, No. 2 (2009)

    doi: 10.1103/PhysRevA.80.022516

    An exact and general approach to study molecular vibrations is provided by the Watson Hamiltonian. Within this framework, it is customary to omit the contribution of the terms with the vibrational angular momentum and the Watson term, especially for the study of large systems. We discover that this omission leads to results which depend on the choice of the reference structure. The self-consistent solution proposed here yields a geometry that coincides with the quantum averaged geometry of the Watson Hamiltonian and appears to be a promising way for the computation of the vibrational spectra of strongly anharmonic systems.

  3. Title: A shell model for the H-bonded ferroelectric KH2PO4

    Author(s): Lasave J., Kohanoff J., Migoni R.L., Koval S.,

    Physica B: Condensed Matter, 404, No. 18, pp. 2736-2738 (2009)

    doi: 10.1016/j.physb.2009.06.143

    A shell model for KH2PO4 (KDP), the prototype compound of the family of H-bonded ferroelectric materials, has been constructed by adjusting the interaction parameters to first-principles calculations. Structural properties, energy barriers, phonons, and the relative stability between the ferroelectric (FE) phase and a relevant antiferroelectric metastable structure associated to domain walls, compare very favorably to available first-principles and experimental data. Molecular dynamics simulations show that the model behaves satisfactorily within the FE phase. This model will be used to study the elusive structure of the paraelectric (PE) phase and the nature of the FE-PE phase transition. (C) 2009 Elsevier B.V. All rights reserved.

  4. Title: Structure of warm dense matter via angularly resolved x-ray scatter

    Author(s): Riley D., Gareta J.J.A., Saiz E.G., Khattak F.Y., Kohanoff J., Sahoo S., Naz G.S., Shearer S.F.C., Thornton K.A., Gregory C., Woolsey N.C., Notley M., Neely D.,

    Plasma Physics and Controlled Fusion, 51, No. 12 (2009)

    doi: 10.1088/0741-3335/51/12/124036

    In this paper we describe experimental results on angularly resolved x-ray scatter from a sample of warm dense aluminium that has been created by double sided laser-driven shock compression. The experiment was carried out on the Central Laser Facility of the Rutherford Appleton Laboratory, using the VULCAN laser. The form of the angularly resolved scatter cross-section was compared with predictions based on a series of molecular dynamics simulations with an embedded atom potential, a Yukakwa potential and a bare Coulomb potential. The importance of screening is evident from the comparison and the embedded atom model seems to match experiment better than the Yukawa potential.