Biomedical subjects

Jean-Paul Malrieu

Publications and source records attributed to Jean-Paul Malrieu.

6 recordsLinked to original sources

PubMed2007

Bond electron pair: its relevance and analysis from the quantum chemistry point of view.

This paper first comments on the surprisingly poor status that Quantum Chemistry has offered to the fantastic intuition of Lewis concerning the distribution of the electrons in the molecule. Then, it advocates in favor of a hierarchical description of the molecular wave-function, distinguishing the physics taking place in the valence space (in the bond and between the bonds), and the dynamical correlation effects. It is argued that the clearest pictures of the valence electronic population combine two localized views, namely the bond (and lone pair) Molecular Orbitals and the Valence Bond decomposition of the wave-function, preferably in the orthogonal version directly accessible from the complete active space self consistent field method. Such a reading of the wave function enables one to understand the work of the nondynamical correlation as an enhancement of the weight of the low-energy VB components, i.e. as a better compromise between the electronic delocalization and the energetic preferences of the atoms. It is suggested that regarding the bond building, the leading dynamical correlation effect may be the dynamical polarization phenomenon. It is shown that most correlation effects do not destroy the bond electron pairs and remain compatible with Lewis' vision. A certain number of free epistemological considerations have been introduced in the development of the argument.

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PubMed2006

First-principles periodic calculation of four-body spin terms in high-Tc cuprate superconductors.

A general mapping between the energy of pertinent magnetic solutions and the diagonal terms of the spin Hamiltonian in a local representation provides the first general framework to extract accurate values for the many body terms of extended spin Hamiltonians from periodic first-principle calculations. Estimates of these terms for La2CuO4, the paradigm of high-Tc superconductor parent compounds, and for the SrCu2O3 ladder compound are reported. For La2CuO4, present results support experimental evidence by Toader et al. [Phys. Rev. Lett. 94, 197202 (2005)10.1103/PhysRevLett.94.197202]. For SrCu2O3 even larger four-body spin amplitudes are found together with Jl/Jr = 1 and non-negligible ferromagnetic interladder exchange.

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PubMed2006

A convenient decontraction procedure of internally contracted state-specific multireference algorithms.

Internally contracted state-specific multireference (MR) algorithms, either perturbative such as CASPT2 or NEVPT2, or nonperturbative such as contracted MR configuration interaction or MR coupled cluster, are computationally efficient but they may suffer from the internal contraction of the wave function in the reference space. The use of a low dimensional multistate model space only offers limited flexibility and is not always practicable. The present paper suggests a convenient state-specific procedure to decontract the reference part of the wave function from a series of state-specific calculations using slightly perturbed zero-order wave functions. The method provides an orthogonal valence bond reading of the ground state and an effective valence Hamiltonian, the excited roots of which are shown to be relevant. The orthogonal valence bond functions can be considered quasidiabatic states and the effective valence Hamiltonian gives therefore the quasidiabatic energies and the electronic coupling among the quasidiabatic states. The efficiency of the method is illustrated in two case problems where the dynamical correlation plays a crucial role, namely, the LiF neutral/ionic avoided crossing and the F(2) ground state wave function.

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PubMed2004

Relative importance of local and collective effects in the distortivity of one-dimensional chains.

On the basis of an original coupled-cluster type formalism developed previously [J.-P. Malrieu and V. Robert, J. Chem. Phys. 120, 7374 (2004)], the cohesive energies and the occurrence of Peierls instability are investigated in half-filled one-dimensional systems. Starting from various parametrizations of the Huckel Hamiltonian, this approach allows one to evaluate the relative contribution of local and collective interactions by comparison to exact tight-binding crystal calculations. For an alternating (AA('))(n) chain, quantitative agreement with the exact solution is obtained starting from either an atom-centered or a bond-centered reference function. The distortion takes place beyond a critical value of the electron-phonon/elastic strength ratio which is correctly predicted. Its amplitude and the corresponding stabilization energy are also accurately reproduced, suggesting that the driving force of the second-order Peierls distortion is essentially local. For homogeneous (A)(n) systems and the first-order Peierls distortion traditionally presented as resulting from a band gap opening (i.e., collective effects), our localized approaches are deficient only in the domain of weak electron-phonon/elastic ratio where the distortion amplitude is almost negligible. These results confirm that the short-range delocalization effects are the leading phenomenon responsible for the bond alternation in conjugated hydrocarbons.

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PubMed2004

A local approach to delocalized electronic systems: semilocal evaluation of the cohesive energies of tight-binding Hamiltonians.

Starting from strongly localized N-electron functions built from either pure atomic orbitals or fully localized bond molecular orbitals, it is possible to evaluate the ground state energy of a periodic lattice ruled by a tight-binding Hamiltonian without explicitly introducing the monoelectronic crystal orbitals. The method consists of a self-consistent perturbation of the zeroth-order wave function which incorporates high order effects and offers reasonable convergence properties. Along this framework, a single variable per bond type is introduced, namely the amplitude of the charge transfer. The method leads to a set of coupled equations which can be numerically solved, if not analytically. Short-range delocalization effects under periodic conditions are explicitly taken into account and relatively accurate cohesive energies are estimated for regular homoatomic and heteroatomic one-dimensional chains as well as for honeycomb lattices. In addition, good agreement with experiment for the distortion amplitude in polyacetylene is obtained. This exploratory tool may be easily extended to more sophisticated Hamiltonians, for which the solutions are not accessible. Since our approach only introduces short-range delocalization effects, its performance questions the importance of the specifically collective delocalization effects.

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PubMed2003

Role of the coordination of the azido bridge in the magnetic coupling of copper(II) binuclear complexes.

It is well-known that the azido bridge gives rise antiferromagnetic (AF) or ferromagnetic (F) coupling depending on its coordination mode, namely end-to-end or end-on, respectively. The aim of the present work is to analyse the factors contributing to this different magnetic behaviour. The difference dedicated configuration interaction (DDCI) method is applied to several binuclear Cu(II) azido-bridged models with both types of coordination. In end-on complexes, the direct exchange and the spin polarisation contributions are found to be responsible for the ferromagnetic coupling. In end-to-end complexes, both the direct exchange and the spin polarisation are small and the leading term is the antiferromagnetic dynamical polarisation contribution. The most relevant physical effects are included in the DDCI calculations so that good quantitative agreement is reached for the coupling constant as well as the spin densities.

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