579 research outputs found
Correlation Energy Estimators based on M{\o}ller-Plesset Perturbation Theory
Some methods for the convergence acceleration of the M{\o}ller-Plesset
perturbation series for the correlation energy are discussed. The
order-by-order summation is less effective than the Feenberg series. The latter
is obtained by renormalizing the unperturbed Hamilton operator by a constant
factor that is optimized for the third order energy. In the fifth order case,
the Feenberg series can be improved by order-dependent optimization of the
parameter. Alternatively, one may use Pad{\'e} approximants or a further method
based on effective characteristic polynomials to accelerate the convergence of
the perturbation series. Numerical evidence is presented that, besides the
Feenberg-type approaches, suitable Pad{\'e} approximants, and also the
effective second order characteristic polynomial, are excellent tools for
correlation energy estimation.Comment: 21 pages, 87 references, LaTeX2e, elsart.cls, uuencoded compressed
tar file created by csh script uufiles. J. Mol. Struct. (THEOCHEM), in press.
Postscript and dvi also available via
http://www.chemie.uni-regensburg.de/preprint.html and
ftp://rchs1.uni-regensburg.de/pub/preprint/TC-QM-96-
Series Prediction based on Algebraic Approximants
It is described how the Hermite-Pad\'e polynomials corresponding to an
algebraic approximant for a power series may be used to predict coefficients of
the power series that have not been used to compute the Hermite-Pad\'e
polynomials. A recursive algorithm is derived and some numerical examples are
given.Comment: 9 pages, ISRN Applied Mathematics, in pres
On Convergence Acceleration of Multipolar and Orthogonal Expansions
Multipolar expansions arise in many branches of the
computational sciences. They are an example of orthogonal expansions.
We present methods for the convergence acceleration of such
expansions. As an example, the computation of the electrostatic potential and its multipolar expansion is treated for the case of a two-center charge
density of exponential-type orbitals. This potential may also be considered as
a special molecular integral, namely as a three-center nuclear attraction
integral. It is shown that in this example, the extrapolation to the limit of the
corresponding expansions via suitable nonlinear sequence transformations leads
to a pronounced convergence acceleration
Spin-Lattice Relaxation in Metal-Organic Platinum(II) Complexes
The dynamics of spin-lattice relaxation (slr) of metal-organic Pt(II)
compounds is studied. Often, such systems are characterized by pronounced
zero-field splittings (zfs) of the lowest-lying triplets. Previous expressions
for the Orbach slr process do not allow to treat such splitting patterns
properly. We discuss the behavior of a modified Orbach expression for a model
system and present results of a fit of the temperature dependence of the
spin-lattice relaxation rate of Pt(2-thpy) based on the modified
expression.Comment: 9 pages, 3 figures (made from 4 .eps files), elsart.cls. Using dvips
(dvipsk 5.58f), it may be necessary to manually edit the generated file
letter.ps to change in the first line from PS-Adobe-2.0 to PS-Adobe-3.0.
Chemical Physics Letters, in pres
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