Biomedical subjects

Anthony Scemama

Publications and source records attributed to Anthony Scemama.

4 recordsLinked to original sources

PubMed2007

Maximum probability domains from Quantum Monte Carlo calculations.

Although it would be tempting to associate the Lewis structures to the maxima of the squared wave function |Psi|2, we prefer in this paper the use of domains of the three-dimensional space, which maximize the probability of containing opposite-spin electron pairs. We find for simple systems (CH4, H2O, Ne, N2, C2H2) domains comparable to those obtained with the electron localization function (ELF) or by localizing molecular orbitals. The different domains we define can overlap, and this gives an interesting physical picture of the floppiness of CH5+ and of the symmetric hydrogen bond in FHF-. The presence of multiple solutions has an analogy with resonant structures, as shown in the trans-bent structure of Si2H2. Correlated wave functions were used (MCSCF or Slater-Jastrow) in the Variational Quantum Monte Carlo framework.

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PubMed2006

An efficient sampling algorithm for variational Monte Carlo.

We propose a new algorithm for sampling the N-body density mid R:Psi(R)mid R:(2)R(3N)mid R:Psimid R:(2) in the variational Monte Carlo framework. This algorithm is based upon a modified Ricci-Ciccotti discretization of the Langevin dynamics in the phase space (R,P) improved by a Metropolis-Hastings accept/reject step. We show through some representative numerical examples (lithium, fluorine, and copper atoms and phenol molecule) that this algorithm is superior to the standard sampling algorithm based on the biased random walk (importance sampling).

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PubMed2004

Electron pair localization function: a practical tool to visualize electron localization in molecules from quantum Monte Carlo data.

In this work we introduce an electron localization function describing the pairing of electrons in a molecular system. This function, called "electron pair localization function," is constructed to be particularly simple to evaluate within a quantum Monte Carlo framework. Two major advantages of this function are the following: (i) the simplicity and generality of its definition; and (ii) the possibility of calculating it with quantum Monte Carlo at various levels of accuracy (Hartree-Fock, multiconfigurational wave functions, valence bond, density functional theory, variational Monte Carlo with explicitly correlated trial wave functions, fixed-node diffusion Monte Carlo, etc). A number of applications of the electron pair localization function to simple atomic and molecular systems are presented and systematic comparisons with the more standard electron localization function of Becke and Edgecombe are done. Results illustrate that the electron pair localization function is a simple and practical tool for visualizing electronic localization in molecular systems.

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PubMed2002

Plasma matrix metalloproteinase-9 as a marker of blood stasis in varicose veins.

BACKGROUND: Possible intermediate circulating markers linking blood stasis to vein remodeling were explored in patients with varicose veins in the lower limbs. METHODS AND RESULTS: Blood was sampled at rest (supine position) and after a stasis of 30 minutes both in the varicose vein (limbs hanging down) and in the brachial vein (arm hanging down) as a paired control. Several endothelial and leukocyte markers were measured in plasma with the use of ELISA kits. Angiotensin-converting enzyme activity was determined by use of a specific substrate. Matrix metalloproteinases (MMPs) 9 and 2 were evaluated with the use of gelatin zymography. No markers were significantly modified after 30 minutes of blood stasis in the brachial vein. After 30 minutes of blood stasis in the varicose vein, oxygen partial pressure decreased (P<0.01). Although thrombomodulin, von Willebrand factor, vascular endothelial growth factor, and MMP-2 were not modified in these conditions, the proteins released by proteolysis from the endothelial membrane intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and angiotensin-converting enzyme were increased (P<0.01). After blood stasis in varicose veins, the leukocyte markers lactoferrin, myeloperoxidase, and interleukin-8 were not modified, whereas L-selectin shed from leukocytes increased (P<0.05), and a major increase in pro-MMP-9, which is released from tertiary granules during polymorphonuclear activation, was observed (P=0.0001). CONCLUSIONS: The marked increase in plasma pro-MMP-9 activity provides evidence of polymorphonuclear activation and granule release in the varicose vein in response to postural blood stasis. Similarly, detection in the plasma of membrane proteins shed from the endothelium or leukocytes provides evidence of pericellular proteolysis.

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