MALRIEU , J.p.

The four-spin operators are suspected to have a significant amplitude and an impact on the collective properties in spin ladders and magnetic two-dimensional lattices. They originate from cyclic circulation of electrons ~ring currents!. Starting from a simple Hubbard Hamiltonian one may establish their form from a fourth order expansion of...

A series of accurate ab initio calculations on CupOq finite clusters, properly embedded in the Madelung potential of the infinite lattice, have been performed in order to determine the local effective interactions in the CuO2 planes of La22xSrxCuO4 compounds. The values of the first-neighbor interactions, magnetic coupling (JNN5125 meV!, and...

This contribution sununarizes sorne of the theoretical strategies developed in the recent past to study systems with an increasing size and complexity by means of accurate extended CI calculations. Sorne examples ofthe capabilities of these strategies are also given, conceming the evaluation of magnetic exchange constants, as well as many ...

The nature of magnetic interactions and electron transfer processes in La22xSrxCuO4 systems are studied, by means of an ab initio embedded cluster model approach, using a difference dedicated configuration interaction ~DDCI! procedure. For the undoped system, the crucial role played by the ligand to metal charge transfer ~LMCT! configurations...

Most interpretations of the magnetic coupling J between two unpaired electrons rest upon simple valence models that involve essentially the ferromagnetic direct exchange contribution, Kab , and the antiferromagnetic effect of the delocalization resulting from the interaction between neutral and ionic determinants, tab , whose energy difference...

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...

Accurate estimates of the magnetic coupling in binuclear complexes can be obtained from ab initio configuration interaction ~CI! calculations using the difference dedicated CI technique. The present paper shows that the same technique also provides a way to analyze the various physical contributions to the coupling and performs numerical...

We present a method that uses the one-particle density matrix to generate directly localized orbitals dedicated to multireference wave functions. On one hand, it is shown that the definition of local orbitals making possible physically justified truncations of the CAS ~complete active space! is particularly adequate for the treatment of...

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...

In magnetic coordination compounds and solids the magnetic orbitals are essentially located on metallic centers but present some delocalization tails on adjacent ligands. Mean field variational calculations optimize this mixing and validate a single band modelization of the intersite magnetic exchange. In this approach, due to the Brillouin's...

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The four-spin cyclic exchange term Jring of three spin-ladder cuprates (SrCu2O3, Sr2Cu3O5, and CaCu2O3) has been calculated from ab initio quantum chemistry calculations. For the first two compounds, a non-negligible cyclic exchange is found, aproximately 20% of the magnetic coupling across the rungs, J⊥, and always larger than the value...

Magnetic interactions in ionic solids are studied using parameter-free methods designed to provide accurate energy differences associated with quantum states defining the Heisenberg constant J. For a series of ionic solids including KNiF3, K2NiF4, KCuF3, K2CuF4, and high- Tc parent compound La2CuO4, the J experimental value is quantitatively...