Center for Molecular Modeling - D.L. Cooper

Comparison of the Hirshfeld-I and iterated stockholder atoms in molecules schemes

wim — Mon, 03 Oct 2011 10:33:26 +0000

P. Bultinck, D.L. Cooper, D. Van Neck

Physical Chemistry Chemical Physics (PCCP)

11 (18), 3424-3429

2009

Abstract

Two recently introduced self-consistent Hirshfeld procedures for obtaining atoms in molecules are compared in detail. The Hirshfeld-I scheme introduces self consistency by requiring that the atomic population of the promolecular atom is equal to that of the atom-in-the-molecule. In the iterated stockholder atoms (ISA) approach, self consistency is obtained by requiring that for every value of the radius of a sphere around every nucleus, the average electron density on the surface of this sphere is the same in the promolecular atom and in the atom in the molecule. The relationships between the two schemes are examined, and common backgrounds and differences are discussed. Whereas it can be argued that the Hirshfeld-I approach has a stronger physical background, the ISA scheme avoids having to define what states of the atoms are to be considered when constructing the promolecule.

DOI

http://dx.doi.org/10.1039/B821734C

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09 phys. chem. chem. phys 11(18)3424 bultinck.pdf

Chemical verification of variational second-order density matrix based potential energy surfaces for the N2 isoelectronic series

wim — Tue, 13 Sep 2011 12:47:47 +0000

H. van Aggelen, B. Verstichel, P. Bultinck, D. Van Neck, P.W. Ayers, D.L. Cooper

Journal of Chemical Physics

132, 114112

2010

Abstract

A variational optimization of the second-order density matrix under the P-, Q-, and G-conditions was carried out for a set of diatomic 14-electron molecules, including N2, O22+, NO+, CO, and CN−. The dissociation of these molecules is studied by analyzing several chemical properties (dipole moments, population analysis, and bond indices) up to the dissociation limit (10 and 20 Å). Serious chemical flaws are observed for the heteronuclear diatomics in the dissociation limit. A careful examination of the chemical properties reveals that the origin of the dissociation problem lies in the flawed description of fractionally occupied species under the P-, Q-, and G-conditions. A novel constraint is introduced that imposes the correct dissociation and enforces size consistency. The effect of this constraint is illustrated with calculations on NO+, CO, CN−, N2, and O22+.

Open Access version available at UGent repository

DOI

http://dx.doi.org/10.1063/1.3354910

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10 j. chem. phys. 132(11)114112 van aggelen.pdf

Variational second order density matrix study of F3−: Importance of subspace constraints for size-consistency

wim — Tue, 23 Aug 2011 07:02:33 +0000

H. van Aggelen, B. Verstichel, P. Bultinck, D. Van Neck, P.W. Ayers, D.L. Cooper

Journal of Chemical Physics

134, 054115

2011

Abstract

Variational second order density matrix theory under “two-positivity” constraints tends to dissociate molecules into unphysical fractionally charged products with too low energies. We aim to construct a qualitatively correct potential energy surface for F3− by applying subspace energy constraints on mono- and diatomic subspaces of the molecular basis space. Monoatomic subspace constraints do not guarantee correct dissociation: the constraints are thus geometry dependent. Furthermore, the number of subspace constraints needed for correct dissociation does not grow linearly with the number of atoms. The subspace constraints do impose correct chemical properties in the dissociation limit and size-consistency, but the structure of the resulting second order density matrix method does not exactly correspond to a system of noninteracting units. © 2011 American Institute of Physics

Open Access version available at UGent repository

DOI

http://dx.doi.org/10.1063/1.3532409

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11_J_ Chem_Phys_134_054115_Van_Aggelen.pdf

Chemical implications of variational second-order density matrix theory: study of diatomic molecules along the potential energy curve

wim — Tue, 18 Oct 2011 06:58:09 +0000

H. van Aggelen, B. Verstichel, P.W. Ayers, P. Bultinck, D.L. Cooper, D. Van Neck

Poster

Conference / event / venue

Workshop on Quantum Marginals and Density Matrices

Toronto, Canada

Monday, 27 July, 2009 to Friday, 31 July, 2009