3,742 research outputs found
Do Community Colleges Respond to Local Needs?: Evidence from California
Leigh and Gill focus on two major, policy relevant sources of change at the local level. First, on the supply side, they examine how responsive community colleges’ are at meeting the needs of the growing immigrant population for education and training. Then, on the demand side, they look into whether the need of local employers for skilled workers is being met, an issue impacted by dynamic technological change and increased global competition.https://research.upjohn.org/up_press/1032/thumbnail.jp
Communication: Efficient counterpoise corrections by a perturbative approach
We investigate the use of Hartree-Fock and density functional perturbative corrections for estimating the counterpoise correction (CPC) for interaction energies at the self-consistent field level. We test our approach using several popular basis sets on the S22 set of weakly bound systems, which can exhibit large basis set superposition errors. Our results show that the perturbative approaches typically recover over 95% of the CPC and can be up to twelve times faster to compute than the conventional methods and therefore provide an attractive alternative to calculating CPCs in the conventional way.P.M.W.G. thanks the Australian Research Council for
funding (Grant Nos. DP0984806 and DP1094170) and APAC
for a generous allocation of supercomputer resources. J.D.
thanks the ANU/RSC for a PhD scholarship
Approaching the Hartree–Fock limit by perturbative methods
We describe perturbative methods for improving finite-basis Hartree-Fock calculations toward the complete-basis limit. The best method appears to offer quadratic error reduction and preliminary numerical applications demonstrate that remarkably accurate Hartree-Fock energies can be obtained.J.D. thanks the ANU for a Ph.D. scholarship and
P.M.W.G. thanks the Australian Research Council for funding
Grant No. DP0771978
MP2[V] – A simple approximation to second-order Møller–Plesset perturbation theory
We propose a simplified variant of the dual-basis MP2[K] scheme [J.
Chem. Phys. 2011,134, 081103] that bootstraps a small-basis MP2 result to a large-basis
one. This simplified method, which we call MP2[V], assumes the occupied orbitals are
adequately described by the smaller basis, and, therefore, only the relaxation of the
virtual orbitals is considered when shifting to the larger basis. Numerical tests on several
organic reactions and noncovalent interactions show that MP2[V] yields absolute and
relative energies that are in excellent agreement with the conventional large-basis MP2
calculations but in a small fraction of the time.P.M.W.G. thanks the Australian Research Council
for funding (Grant Nos. DP1094170 and DP120104740) and
APAC for a generous allocation of supercomputer resources.
J.D. thanks the ANU/RSC for a Ph.D. scholarship
Hartree–Fock perturbative corrections for total and reaction energies
We have performed an assessment of the Hartree-Fock perturbative correction (HFPC) on a large and diverse set of molecules and reactions. Errors in both absolute and reaction energies with respect to converged secondary basis Hartree-Fock results are reported for a wide spectrum of primary/secondary basis set combinations. These results show that using an adequate primary basis, HFPC can accurately reproduce secondary basis energies at a substantially reduced cost. Comparisons of HFPC with the related dual basis Hartree-Fock (DBHF) scheme are also made for several molecules and target secondary basis sets. Our results indicate that HFPC is faster and more accurate than DBHF for approaching triple-zeta basis sets. For quadruple-zeta secondary basis sets, HFPC is capable of yielding more accurate energies at a marginally increased cost over DBHF.We thank the Australian Research Council for funding
Grant No. DP0771978 and APAC for a generous allocation
of supercomputer resources. J.D. thanks the ANU/RSC for a Ph.D. scholarship
A generalized Poisson equation and short-range self-interaction energies
We generalize the Poisson equation to attenuated Newtonian potentials. If the attenuation is at least exponential, the equation provides a local mapping between the density and its potential. We use this to derive several density functionals for the short-range self-interaction energy
Self-consistent-field calculations of core excited states
The accuracy of core excitation energies and core electron binding energies computed within a Δself-consistent-field framework is assessed. The variational collapse of the core excited state is prevented by maintaining a singly occupied core orbital using an overlap criterion called the maximum overlap method. When applied to a wide range of small organic molecules, the resulting core excitation energies are not systematically underestimated as observed in time-dependent density functional theory and agree well with experiment. The accuracy of this approach for core excited states is illustrated by the calculation of the pre-edge features in x-ray absorption spectra of plastocyanin, which shows that accurate results can be achieved with Δself-consistent-field calculations when used in conjunction with uncontracted basis functions.N.A.B. is grateful to the ANU for a 2007 Visiting
Fellowship
Extracting atoms from molecular electron densities via integral equations
The observation that a molecular electron density is close to the superposition of its constituent atoms leads naturally to the idea of modeling a density by a sum of nuclear-centered, spherically symmetric functions. The functions that are optimal in a least-squares sense are known as Stewart atoms. Previous attempts to construct Stewart atoms by expanding them in an auxiliary basis have been thwarted by slow convergence with respect to the size of the auxiliary basis used. We present a method for constructing Stewart atoms via convolution integrals which bypasses the need for an auxiliary basis, and is able to produce highly accurate approximations to Stewart atoms.ATBG is grateful for funding and support from QCHEM
Inc
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