Fock Calculations (fock + calculation)

Distribution by Scientific Domains


Selected Abstracts


Starting SCF calculations by superposition of atomic densities

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2006
J. H. Van Lenthe
Abstract We describe the procedure to start an SCF calculation of the general type from a sum of atomic electron densities, as implemented in GAMESS-UK. Although the procedure is well known for closed-shell calculations and was already suggested when the Direct SCF procedure was proposed, the general procedure is less obvious. For instance, there is no need to converge the corresponding closed-shell Hartree,Fock calculation when dealing with an open-shell species. We describe the various choices and illustrate them with test calculations, showing that the procedure is easier, and on average better, than starting from a converged minimal basis calculation and much better than using a bare nucleus Hamiltonian. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 926,932, 2006 [source]


Soft Coulomb hole method applied to molecules

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2007
J. Ortega-Castro
Abstract The soft Coulomb hole method introduces a perturbation operator, defined by ,e/r12 to take into account electron correlation effects, where , represents the width of the Coulomb hole. A new parametrization for the soft Coulomb hole operator is presented with the purpose of obtaining better molecular geometries than those resulting from Hartree,Fock calculations, as well as correlation energies. The 12 parameters included in , were determined for a reference set of 12 molecules and applied to a large set of molecules (38 homo- and heteronuclear diatomic molecules, and 37 small and medium-size molecules). For these systems, the optimized geometries were compared with experimental values; correlation energies were compared with results of the MP2, B3LYP, and Gaussian 3 approach. On average, molecular geometries are better than the Hartree,Fock values, and correlation energies yield results halfway between MP2 and B3LYP. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source]


Radiative transitions of the helium atom in highly magnetized neutron star atmospheres

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Z. Medin
ABSTRACT Recent observations of thermally emitting isolated neutron stars revealed spectral features that could be interpreted as radiative transitions of He in a magnetized neutron star atmosphere. We present Hartree,Fock calculations of the polarization-dependent photoionization cross-sections of the He atom in strong magnetic fields ranging from 1012 to 1014 G. Convenient fitting formulae for the cross-sections are given along with the related oscillator strengths for various bound,bound transitions. The effects of finite nucleus mass on the radiative absorption cross-sections are examined using perturbation theory. [source]


Electron localization functions obtained from X-ray constrained Hartree,Fock wavefunctions for mol­ecular crystals of ammonia, urea and alloxan,

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2004
Dylan Jayatilaka
Constrained Hartree,Fock wavefunctions in the superposition of isolated molecules model have been calculated from precise X-ray diffraction data on crystals of ammonia (NH3), urea [CO(NH2)2] and alloxan [(CO)4(NH)2]. The X-ray constrained wavefunctions have been used to derive and examine `electron localization' information, quantified by the true and approximate electron localization function (ELF), and the true and approximate Fermi hole mobility function (FHMF). The plots of the Fermi hole mobility function are the first to appear in the literature. The results are compared with corresponding isolated-molecule Hartree,Fock calculations to gauge the effect of the crystal environment on the isolated molecules. An error analysis is performed to indicate the features in the plots which are well determined from the experimental data. The results from all plots are broadly consistent, but the approximate ELF shows some artifacts relative to the true ELF. [source]


6,-Hydroxy-5,-methyl-20-oxo-19-norpregn-9(10)-en-3,-yl acetate

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2009
R. M. A. Pinto
In the title compound, C23H34O4, which is an intermediate in the synthesis of pregnane derivatives with a modified skeleton that show potent abortion-inducing activity, the conformation of ring B is close to half-chair due to the presence of both the C=C double bond and the axial 5,-methyl group. Rings A and C have conformations close to chair, while ring D has a twisted conformation around the bridgehead C,C bond. Molecules are hydrogen bonded via the hydroxyl and acetoxy groups into infinite chains. Quantum-mechanical ab initio Roothan Hartree,Fock calculations show that crystal packing might be responsible for the low values of the angles between rings A and B, and between ring A and rings C and D, as well as for a different steric position of the methyl ketone side chain compared to the geometry of the free molecule. [source]