33,874 research outputs found
Assessment of W1 and W2 theories for the computation of electron affinities, ionization potentials, heats of formation, and proton affinities
The performance of two recent {\em ab initio} computational thermochemistry
schemes, W1 and W2 theory [J.M.L. Martin and G. de Oliveira, J. Chem. Phys.
111, 1843 (1999}], is assessed for an enlarged sample of thermochemical data
consisting of the ionization potentials and electron affinities in the G2-1 and
G2-2 sets, as well as the heats of formation in the G2-1 and a subset of the
G2-2 set. We find W1 theory to be several times more accurate for ionization
potentials and electron affinities than commonly used (and less expensive)
computational thermochemistry schemes such as G2, G3, and CBS-QB3: W2 theory
represents a slight improvement for electron affinities but no significant one
for ionization potentials. The use of a two-point rather than a
three-point extrapolation for the SCF component greatly enhances the
numerical stability of the W1 method for systems with slow basis set
convergence. Inclusion of first-order spin-orbit coupling is essential for
accurate ionization potentials and electron affinities involving degenerate
electronic states: inner-shell correlation is somewhat more important for
ionization potentials than for electron affinities, while scalar relativistic
effects are required for the highest accuracy. The mean deviation from
experiment for the G2-1 heats of formation is within the average experimental
uncertainty. W1 theory appears to be a valuable tool for obtaining benchmark
quality proton affinities.Comment: Journal of Chemical Physics, in press (303115JCP). 2 RevTeX files,
first is text and tables, second is E-PAPS tables S-1 through S-5. Additional
supplementary material (total energies, basis function exponents) available
at http://theochem.weizmann.ac.il/web/papers/w1w2.htm
Some Observations on the Performance of the Most Recent Exchange-Correlation Functionals for the Large and Chemically Diverse GMTKN55 Benchmark
Benchmarks that span a broad swath of chemical space, such as GMTKN55, are
very useful for assessing progress in the quest for more universal DFT
functionals. We find that the WTMAD2 metrics for a great number of functionals
show a clear "Jacob's Ladder hierarchy"; that the "combinatorial" development
strategy of Head-Gordon and coworkers generates "best on rung" performers; that
the quality of the nonlocal dispersion correction becomes more important as
functionals become more accurate for nondispersion properties; that fitting
against small, unrepresentative benchmark sets leads to underperforming
functionals; and that {\omega}B97M(2) is currently the best DFT functional of
any kind, but that revDSD-D4 functionals are able to reach similar performance
using fewer parameters, and that revDOD-D4 in addition permits reduced-scaling
algorithms. If one seeks a range-separated hybrid (RSH) GGA that also performs
well for optical excitation energies, CAM-QTP-01 may be a viable option. The D4
dispersion model, with its partial charge dependence, appears to be clearly
superior to D3BJ and even possibly NL. Should one require a double hybrid
without dispersion model, noDispSD-SCAN is a viable option. Performance for the
MOBH35 transition metal benchmark is different: the best double hybrids are
competitive but not superior to {\omega}B97M-V, which offers the best
performance compromise for mixed main group-transition metal problems.Comment: 5 pages (ICCMSE-2019 conference proceedings), AIP Conference
Proceedings, in pres
A definitive heat of vaporization of silicon through benchmark ab initio calculations on SiF_4
In order to resolve a significant uncertainty in the heat of vaporization of
silicon -- a fundamental parameter in gas-phase thermochemistry -- [Si(g)] has been determined from a thermochemical cycle involving
the precisely known experimental heats of formation of SiF_4(g) and F(g) and a
benchmark calculation of the total atomization energy (TAE_0) of SiF_4 using
coupled-cluster methods. Basis sets up to on Si and
on F have been employed, and extrapolations for residual basis
set incompleteness applied. The contributions of inner-shell correlation (-0.08
kcal/mol), scalar relativistic effects (-1.88 kcal/mol), atomic spin-orbit
splitting (-1.97 kcal/mol), and anharmonicity in the zero-point energy (+0.04
kcal/mol) have all been explicitly accounted for. Our benchmark TAE_0=565.89
\pm 0.22 kcal/mol leads to [Si(g)]=107.15 \pm 0.38
kcal/mol ([Si(g)]=108.19 \pm 0.38 kcal/mol): between
the JANAF/CODATA value of 106.5 \pm 1.9 kcal/mol and the revised value proposed
by Grev and Schaefer [J. Chem. Phys. 97, 8389 (1992}], 108.1 \pm 0.5 kcal/mol.
The revision will be relevant for future computational studies on heats of
formation of silicon compounds.Comment: J. Phys. Chem. A, submitted Feb 1, 199
Development of Novel Density Functionals for Thermochemical Kinetics
A new density functional theory (DFT) exchange-correlation functional for the
exploration of reaction mechanisms is proposed. This new functional, denoted
BMK (Boese-Martin for Kinetics), has an accuracy in the 2 kcal/mol range for
transition state barriers but, unlike previous attempts at such a functional,
this improved accuracy does not come at the expense of equilibrium properties.
This makes it a general-purpose functional whose domain of applicability has
been extended to transition states, rather than a specialized functional for
kinetics. The improvement in BMK rests on the inclusion of the kinetic energy
density together with a large value of the exact exchange mixing coefficient.
For this functional, the kinetic energy density appears to correct `back' the
excess exact exchange mixing for ground-state properties, possibly simulating
variable exchange.Comment: J. Chem. Phys., in press (303431JCP, scheduled for August 15, 2004
issue); supplementary data available at
http://theochem.weizmann.ac.il/web/papers/BMK.htm
Towards standard methods for benchmark quality ab initio thermochemistry --- W1 and W2 theory
Two new schemes for computing molecular total atomization energies (TAEs)
and/or heats of formation () of first-and second-row
compounds to very high accuracy are presented. The more affordable scheme, W1
(Weizmann-1) theory, yields a mean absolute error of 0.30 kcal/mol and includes
only a single, molecule-independent, empirical parameter. It requires CCSD
(coupled cluster with all single and double substitutions) calculations in
and basis sets, while CCSD(T) [i.e. CCSD with a
quasiperturbative treatment of connected triple excitations] calculations are
only required in and basis sets. On workstation computers and
using conventional coupled cluster algorithms, systems as large as benzene can
be treated, while larger systems are feasible using direct coupled cluster
methods. The more rigorous scheme, W2 (Weizmann-2) theory, contains no
empirical parameters at all and yields a mean absolute error of 0.23 kcal/mol,
which is lowered to 0.18 kcal/mol for molecules dominated by dynamical
correlation. It involves CCSD calculations in and basis sets
and CCSD(T) calculations in and basis sets. On workstation
computers, molecules with up to three heavy atoms can be treated using
conventional coupled cluster algorithms, while larger systems can still be
treated using a direct CCSD code. Both schemes include corrections for scalar
relativistic effects, which are found to be vital for accurate results on
second-row compounds.Comment: J. Chem. Phys., in press; text 30 pages RevTeX; tables 10 pages, HTML
and PostScript versions both included Reason for replacement: fixed typos in
Table II in proo
Results of the 2004/2005 Hospice Patient Survey: General Report
The National Minimum Standards for Independent Healthcare, published in 2002 by the Care Standards Commission (now the Healthcare Commission) states that hospices are required to conduct an annual patient survey. In response to this, a Patient Survey group (with representation from independent hospices, Marie Curie Cancer Care and with input from the Care Standards Commission) was set up by Help the Hospices. Its goal was to develop a questionnaire suitable for use in all adult hospices
The exact power envelope of tests for a unit root
We show how to obtain the exact power envelope of tests for a unit root against trend-stationary alternatives, under normality. This is in contrast to the asymptotic power envelope derived by Elliott, Rothenberg and Stock (1996), and is used to indicate the lack of power of unit root tests in fixed sample sizes. <br><br> Keywords; power envelope, unit root tests <br><br> JEL classification: C12, C22
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