7,276 research outputs found
Correlation energy of two electrons in the high-density limit
We consider the high-density-limit correlation energy \Ec in
dimensions for the ground states of three two-electron systems: helium
(in which the electrons move in a Coulombic field), spherium (in which they
move on the surface of a sphere), and hookium (in which they move in a
quadratic potential). We find that the \Ec values are strikingly similar,
depending strongly on but only weakly on the external potential. We
conjecture that, for large , the limiting correlation energy \Ec \sim
-\delta^2/8 in any confining external potential, where .Comment: 4 pages, 0 figur
Excitation Gap from Optimized Correlation Functions in Quantum Monte Carlo Simulations
We give a prescription for finding optimized correlation functions for the
extraction of the gap to the first excited state within quantum Monte Carlo
simulations. We demonstrate that optimized correlation functions provide a more
accurate reading of the gap when compared to other `non-optimized' correlation
functions and are generally characterized by considerably larger
signal-to-noise ratios. We also analyze the cost of the procedure and show that
it is not computationally demanding. We illustrate the effectiveness of the
proposed procedure by analyzing several exemplary many-body systems of
interacting spin-1/2 particles.Comment: 11 pages, 5 figure
Ab-initio-MO-Studie Methyl- und Phenyl-substituierter Allenyl-Kationen
An den Methyl- und Phenyl-substituierten Allenyl-Kationen 3 - 12 (Tab. 1) wurden ab-initio-MO-Berechnungen unter Verwendung des STO-3G Basissatzes durchgefĂŒhrt. Die berechneten BindungslĂ€ngen und Ladungsverteilungen zeigen Delokalisierung der positiven Ladung an, wie in Formel 1 gezeigt. Mit Hilfe isodesmischer Reaktionen werden Stabilisierungsenergien von Substituenten in 1- und 3-Position ermittelt. Diese Werte ermöglichen in Kombination mit der experimentell bekannten BildungswĂ€rme des Stammkörpers 2 die Bestimmung von H sĂ€mtlicher Allenyl-Kationen 3 - 12. Der Vergleich dieser Daten mit einigen experimentell bestimmten BildungswĂ€rmen zeigt Ăbereinstimmung innerhalb von 2 kcal/mol. Es werden Voraussagen fĂŒr das Reaktionsverhalten gegenĂŒber n-Nucleophilen und -Systemen gemacht
Representations of molecular force fields. I. Ethane: Ab initio and model, harmonic and anharmonic
The quadratic and selected cubic force constants for ethane have been computed, using single determinant molecular orbital wavefunctions at the 4â31G level, with a view to testing and extending model consistent force fields (CFF) for ââmolecular mechanicsââ calculations. Results agree semiquantitatively with experiment, but experimental force constants of sufficient reliability to provide a definitive comparison are not yet available. In a comparison with the most rational general CFF available, that of Ermer and Lifson, the most significant discrepancies found to occur are those for certain stretchâbend couplings assumed to be zero in the CFF but shown to be appreciable by quantum calculation. It is observed that these couplings, but not the stretchâstretch couplings, are well accounted for by a steric interaction model. The ab initio cubic constants examined display the same pattern of conformity with a steric model. Bendâbendâbend and bendâbendâstretch but not all stretchâstretchâstretch interactions agree with those of the steric model. The partial success of the steric model shows that it is possible to represent a large number of interaction constants, quadratic and higher order, by a small number of parameters in molecular mechanics. The failure of the steric model to account for predominantly stretching interactions confirms that ââclassicalââ nonbonded interactions as embodied in conventional UreyâBradley fields are not the only major contributors to offâdiagonal force constants. An alternative model, the anharmonic model of Warshel, as modified by Kirtman et al., was found to account well for pure stretches but not for bends or stretchâbend interactions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70643/2/JCPSA6-63-11-4750-1.pd
- âŠ