Jesús Cabrero

Analysis of the magnetic coupling in binuclear complexes. I. Physics of the coupling

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 analysis of their respective roles on four binuclear complexes of Cu (d9) ions. The bare valence-only description (including direct and kinetic exchange) does not result in meaningful values. The spin-polarization phenomenon cannot be neglected, its sign and amplitude depend on the system. The two leading dynamical correlation effects have an antiferromagnetic character. The first one goes through the dynamical polarization of the environment in the ionic valence bond forms (i.e., the M+⋯M− structures). The second one is due to the double excitations involving simultaneously single excitations between the bridging ligand and the magnetic orbitals and single excitations of...

Analysis of the magnetic coupling in binuclear complexes. II. Derivation of valence effective Hamiltonians from ab initio CI and DFT calculations

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 is U. Ab initio valence-only calculations give very poor estimates of J, whatever the definition of the magnetic orbitals, and large CI expansions are required to evaluate it properly. It is, however, possible to define valence effective Hamiltonians from the knowledge of the eigenenergies and the eigenvectors of these accurate CI calculations. When applied to four different complexes, this strategy shows that spin polarization may change the sign of the direct exchange interaction, Kab, and that dynamical correlation results in a dramatic reduction of the effective repulsion U. The present article also shows how Kab, tab, and U effective parameters can be extracted f...

Ferrimagnetic coupling in oxamido-bridged Mn(II)Cu(II) compounds: a combined CASPT2 and DDCI study

Multiconfigurational perturbation theory (CASPT2) and difference dedicated configuration interaction (DDCI) are applied to study the ferrimagnetic coupling in an oxamido-bridged Mn(II)Cu(II) molecular species. CASPT2 reproduces the experimental coupling very well. From the partition of the CASPT2 energy, the most important contributions to the coupling are established. Spin populations are calculated with DDCI. The successive improvement of the N-electron wave function allows us to analyse the contributions to the spin delocalization.

Efficiency of a controlled use of Davidson's correction for the calculation of excitation energies: vertical spectrum of trans-butadiene

As illustrated using the difficult problem of the trans-butadiene excitation spectrum, the use of the generalized Davidsons correction for the multireference CI description gives very reliable results when using the difference dedicated configuration interaction scheme, provided that the zero-order descriptions of the states involved are as good as possible and of similar quality. This balanced improvement is obtained by calculating average density matrices and using iteratively their mean natural orbitals. The results become quite stable when changing the size of the zero-order complete active spaces, and are in agreement with experimental values within a range of 0.1 eV.

Magnetic coupling in oxalato-bridged hetero-bimetallic compounds: an ab initio study

Abstract Multiconfigurational perturbation theory (CASPT2) and difference dedicated configuration interaction are applied to study the ferromagnetic coupling in oxalato-bridged Cr(III)Cu(II) molecular species, where the external ligands of the Cr(salen)C2O4Cu(acpy) complex, with salen=N,N′-ethylenebis(salicylideneaminate) and acpy=N-acetylacetonylidene-N-(2-pyridylethyl)aminate, are modeled by simple H2O or NH3 groups. The results show little dependence on the external ligand modeling. The experimental coupling is very well reproduced at both calculation levels when the model nearly reproduces the experimental geometrical structure. CASPT2 calculations on an oxalato-bridged Cr(III)Ni(II) model of Cr(salen)C2O4Ni(taea)BPh4, where taea=tris(2-aminoethyl)amine, are also in good agreement with experiment.