The excitation energies and ionization potentials of the atoms in the first transition series are notoriously difficult to compute accurately. Errors in calculated excitation energies can range from 1-4 eV at the Hartree-Fock level, and errors as high as 1.5eV are encountered for ionization energies. In the current work we present and discuss the results of a systematic study of the first transition series using a spin-restricted Kohn-Sham density-functional method with the gradient-corrected functionals of Becke and Lee, Yang and Parr. Ionization energies are observed to be in good agreement with experiment, with a mean absolute error of approximately 0.15eV; these results are comparable to the most accurate calculations to date, the Quadratic Configuration Interaction (QCISD(T)) calculations of Raghavachari and Trucks. Excitation energies are calculated with a mean error of approximately 0.5eV, compared with ∼ 1eV for the local density approximation and 0.1eV for QCISD(T). These gradient-corrected functionals appear to offer an attractive compromise between accuracy and computational effort.
We describe improved algorithms for carrying out pseudospectral Hartree–Fock calculations; these algorithms are applicable to other ab initio electronic structure methodologies as well. Absolute energies agree with conventional basis set codes to within 0.25 kcal/mol, and relative energies agree to better than 0.1 kcal/mol for a wide variety of test molecules. Accelerations of CPU times of as large as a factor of 6.5 are obtained as compared to GAUSSIAN 92, with the actual timing advantage increasing for larger basis sets and larger molecules. The method is shown to be highly reliable and capable of handling extended basis sets.
The relative reaction thermodynamics for the probable pathways
leading to alkene
epoxidation, cis-dihydroxylation, and C−C cleavage as well
as alkenylation of aldehydes
have been investigated for the reaction of ethylene with
LReO3, L = CH3, Cl, OH,
OCH3,
O-, C5H5(Cp), and
C5(CH3)5 (Cp*). We find
the reaction pathway exothermicities to be quite
sensitive to the ligand. For ClReO3,
CH3ReO3, HOReO3,
CH3OReO3, and
O-ReO3, reaction
with ethylene is endothermic with metallaoxetane preferred over
dioxylate formation. For
CpReO3 and Cp*ReO3, the reaction with ethylene
is exothermic but nearly thermoneutral
with dioxylate formation preferred. The π-binding attributes of
the ligand are found to
correlate with thermodyanmic preference.
This document is a reference guide to the Xyce Parallel Electronic Simulator, and is a companion document to the Xyce Users' Guide. The focus of this document is (to the extent possible) exhaustively list device parameters, solver options, parser options, and other usage details of Xyce. This document is not intended to be a tutorial. Users who are new to circuit simulation are better served by the Xyce Users' Guide. Trademarks The information herein is subject to change without notice.
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Density functional theory(DFT) and Hartree-Fock(HF) calculations are reported for the family of transition metal fluorides ScF 3 , TiF 4 , VF 5 , and CrF 6 . Both HF and the local-density-aproximation (LDA) yield excellent agreement with experimental bond lengths, while the B-LYP gradient-corrected density functional gives bond lengths 0.04 − 0.05Å too long. An investigation of various combinations of exchange and correlation functionals shows that, for this series, the origin of this behavior lies in the Becke exchange functional. Much improved bond distances are found using the hybrid HF/DFT functional advocated by Becke. This approximation also leads to much improved thermochemistries. The LDA overestimates average bond energies in this series by 30 − 40 kcal/mol, whereas the B-LYP functional overbinds by only ∼ 8 − 12 kcal/mol, and the hybrid HF/DFT method overbinds by only ∼ 2 kcal/mol. The hybrid method predicts the octahedral isomer of CrF 6 to be more stable than the trigonal prismatic form by 14 kcal/mol. Comparison of theoretical vibrational frequencies with experiment supports the assignment of an octahedral geometry.
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