50 research outputs found
Classical Langevin Dynamics for Model Hamiltonians
We propose a scheme for extending the model Hamiltonian method developed
originally for studying the equilibrium properties of complex perovskite
systems to include Langevin dynamics. The extension is based on Zwanzig's
treatment of nonlinear generalized Langevin's equations. The parameters
entering the equations of motion are to be determined by mapping from
first-principles calculations, as in the original model Hamiltonian method. The
scheme makes possible, in principle, the study of the dynamics and kinetics of
structural transformations inaccessible to the original model Hamiltonian
method. Moreover, we show that the equilibrium properties are governed by an
effective Hamiltonian which differs from that used in previous work by a term
which captures the coherent part of the previously ignored dynamical
interaction with the omitted degrees of freedom. We describe how the additional
information required for the Langevin equations can be obtained by a minor
extension of the previous mapping.Comment: 5 pages, no figures, to appear in Physica Status Solidi; replacement
acknowledges funding agenc
Monoclinic and triclinic phases in higher-order Devonshire theory
Devonshire theory provides a successful phenomenological description of many
cubic perovskite ferroelectrics such as BaTiO3 via a sixth-order expansion of
the free energy in the polar order parameter. However, the recent discovery of
a novel monoclinic ferroelectric phase in the PZT system by Noheda et al.
(Appl. Phys. Lett. 74, 2059 (1999)) poses a challenge to this theory. Here, we
confirm that the sixth-order Devonshire theory cannot support a monoclinic
phase, and consider extensions of the theory to higher orders. We show that an
eighth-order theory allows for three kinds of equilibrium phases in which the
polarization is confined not to a symmetry axis but to a symmetry plane. One of
these phases provides a natural description of the newly observed monoclinic
phase. Moreover, the theory makes testable predictions about the nature of the
phase boundaries between monoclinic, tetragonal, and rhombohedral phases. A
ferroelectric phase of the lowest (triclinic) symmetry type, in which the
polarization is not constrained by symmetry, does not emerge until the
Devonshire theory is carried to twelfth order. A topological analysis of the
critical points of the free-energy surface facilitates the discussion of the
phase transition sequences.Comment: 10 pages, with 5 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/dv_pzt/index.htm
Partition theory: A very simple illustration
We illustrate the main features of a recently proposed method based on
ensemble density functional theory to divide rigorously a complex molecular
system into its parts [M.H. Cohen and A. Wasserman, J. Phys. Chem. A 111, 2229
(2007)]. The illustrative system is an analog of the hydrogen molecule for
which analytic expressions for the densities of the parts (hydrogen "atoms")
are found along with the "reactivity potential" that enters the theory. While
previous formulations of Chemical Reactivity Theory lead to zero, or undefined,
values for the chemical hardness of the isolated parts, we demonstrate they can
acquire a finite and positive hardness within the present formulation.Comment: 8 pages, 8 figure
A well-scaling natural orbital theory
We introduce an energy functional for ground-state electronic structure
calculations. Its variables are the natural spin-orbitals of singlet many-body
wave functions and their joint occupation probabilities deriving from
controlled approximations to the two-particle density matrix that yield
algebraic scaling in general, and Hartree-Fock scaling in its seniority-zero
version. Results from the latter version for small molecular systems are
compared with those of highly accurate quantum-chemical computations. The
energies lie above full configuration interaction calculations, close to doubly
occupied configuration interaction calculations. Their accuracy is considerably
greater than that obtained from current density-functional theory
approximations and from current functionals of the one-particle density matrix.Comment: http://www.pnas.org/cgi/doi/10.1073/pnas.1615729113. arXiv admin
note: text overlap with arXiv:1309.392
Total energy density as an interpretative tool
We present an unambiguous formulation for the total energy density within
density-functional theory. We propose that it be used as a tool for the
interpretation of computed energy and electronic structure changes during
structural transformations and chemical reactions, augmenting the present use
of electron density changes and changes in the Kohn-Sham local density of
states and Kohn-Sham energy density.Comment: 5 pages, 3 embedded figures, submitted to J. Chem. Phy
Extrinsic models for the dielectric response of CaCu{3}Ti{4}O{12}
The large, temperature-independent, low-frequency dielectric constant
recently observed in single-crystal CaCu{3}Ti{4}O{12} is most plausibly
interpreted as arising from spatial inhomogenities of its local dielectric
response. Probable sources of inhomogeneity are the various domain boundaries
endemic in such materials: twin, Ca-ordering, and antiphase boundaries. The
material in and neighboring such boundaries can be insulating or conducting. We
construct a decision tree for the resulting six possible morphologies, and
derive or present expressions for the dielectric constant for models of each
morphology. We conclude that all six morphologies can yield dielectric behavior
consistent with observations and suggest further experiments to distinguish
among them.Comment: 9 pages, with 1 postscript figure embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/mc_ext/index.htm