814 research outputs found
Condensation and Slow Dynamics of Polar Nanoregions in Lead Relaxors
It is now well established that the unique properties of relaxor
ferroelectrics are due to the presence of polar nanoregions (PNR's). We present
recent results from Neutron and Raman scattering of single crystals of PZN,
PZN-xPT, and PMN. Both sets of measurements provide information on the
condensation of the PNR's and on their slow dynamics, directly through the
central peak and, indirectly, through their coupling to transverse phonons. A
comparative analysis of these results allows identification of three stages in
the evolution of the PNR's with decreasing temperature: a purely dynamic stage,
a quasi-static stage with reorientational motion and a frozen stage. A model is
proposed, based on a prior study of KTN, which explains the special behavior of
the transverse phonons (TO and TA) in terms of their mutual coupling through
the rotations of the PNR's.Comment: AIP 6x9 style files, 10 pages, 4 figures, Conference-Fundamental
Physics of Ferroelectrics 200
Optimization of quantum Monte Carlo wave functions by energy minimization
We study three wave function optimization methods based on energy
minimization in a variational Monte Carlo framework: the Newton, linear and
perturbative methods. In the Newton method, the parameter variations are
calculated from the energy gradient and Hessian, using a reduced variance
statistical estimator for the latter. In the linear method, the parameter
variations are found by diagonalizing a non-symmetric estimator of the
Hamiltonian matrix in the space spanned by the wave function and its
derivatives with respect to the parameters, making use of a strong
zero-variance principle. In the less computationally expensive perturbative
method, the parameter variations are calculated by approximately solving the
generalized eigenvalue equation of the linear method by a nonorthogonal
perturbation theory. These general methods are illustrated here by the
optimization of wave functions consisting of a Jastrow factor multiplied by an
expansion in configuration state functions (CSFs) for the C molecule,
including both valence and core electrons in the calculation. The Newton and
linear methods are very efficient for the optimization of the Jastrow, CSF and
orbital parameters. The perturbative method is a good alternative for the
optimization of just the CSF and orbital parameters. Although the optimization
is performed at the variational Monte Carlo level, we observe for the C
molecule studied here, and for other systems we have studied, that as more
parameters in the trial wave functions are optimized, the diffusion Monte Carlo
total energy improves monotonically, implying that the nodal hypersurface also
improves monotonically.Comment: 18 pages, 8 figures, final versio
Zero-variance zero-bias quantum Monte Carlo estimators of the spherically and system-averaged pair density
We construct improved quantum Monte Carlo estimators for the spherically- and
system-averaged electron pair density (i.e. the probability density of finding
two electrons separated by a relative distance u), also known as the
spherically-averaged electron position intracule density I(u), using the
general zero-variance zero-bias principle for observables, introduced by
Assaraf and Caffarel. The calculation of I(u) is made vastly more efficient by
replacing the average of the local delta-function operator by the average of a
smooth non-local operator that has several orders of magnitude smaller
variance. These new estimators also reduce the systematic error (or bias) of
the intracule density due to the approximate trial wave function. Used in
combination with the optimization of an increasing number of parameters in
trial Jastrow-Slater wave functions, they allow one to obtain well converged
correlated intracule densities for atoms and molecules. These ideas can be
applied to calculating any pair-correlation function in classical or quantum
Monte Carlo calculations.Comment: 13 pages, 9 figures, published versio
Compact and Flexible Basis Functions for Quantum Monte Carlo Calculations
Molecular calculations in quantum Monte Carlo frequently employ a mixed basis
consisting of contracted and primitive Gaussian functions. While standard basis
sets of varying size and accuracy are available in the literature, we
demonstrate that reoptimizing the primitive function exponents within quantum
Monte Carlo yields more compact basis sets for a given accuracy. Particularly
large gains are achieved for highly excited states. For calculations requiring
non-diverging pseudopotentials, we introduce Gauss-Slater basis functions that
behave as Gaussians at short distances and Slaters at long distances. These
basis functions further improve the energy and fluctuations of the local energy
for a given basis size. Gains achieved by exponent optimization and
Gauss-Slater basis use are exemplified by calculations for the ground state of
carbon, the lowest lying excited states of carbon with , ,
, symmetries, carbon dimer, and naphthalene. Basis size
reduction enables quantum Monte Carlo treatment of larger molecules at high
accuracy.Comment: 8 Pages, 2 Figures, 9 Table
Approaching Chemical Accuracy with Quantum Monte Carlo
A quantum Monte Carlo study of the atomization energies for the G2 set of
molecules is presented. Basis size dependence of diffusion Monte Carlo
atomization energies is studied with a single determinant Slater-Jastrow trial
wavefunction formed from Hartree-Fock orbitals. With the largest basis set, the
mean absolute deviation from experimental atomization energies for the G2 set
is 3.0 kcal/mol. Optimizing the orbitals within variational Monte Carlo
improves the agreement between diffusion Monte Carlo and experiment, reducing
the mean absolute deviation to 2.1 kcal/mol. Moving beyond a single determinant
Slater-Jastrow trial wavefunction, diffusion Monte Carlo with a small complete
active space Slater-Jastrow trial wavefunction results in near chemical
accuracy. In this case, the mean absolute deviation from experimental
atomization energies is 1.2 kcal/mol. It is shown from calculations on systems
containing phosphorus that the accuracy can be further improved by employing a
larger active space.Comment: 6 pages, 5 figure
Alleviation of the Fermion-sign problem by optimization of many-body wave functions
We present a simple, robust and highly efficient method for optimizing all
parameters of many-body wave functions in quantum Monte Carlo calculations,
applicable to continuum systems and lattice models. Based on a strong
zero-variance principle, diagonalization of the Hamiltonian matrix in the space
spanned by the wav e function and its derivatives determines the optimal
parameters. It systematically reduces the fixed-node error, as demonstrated by
the calculation of the binding energy of the small but challenging C
molecule to the experimental accuracy of 0.02 eV
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