Evaluation of magnetic terms in Cu4O4 cubane-like systems from selected configuration interaction calculations: A case study of polynuclear transition-metal systems

We present the evaluation of magnetic terms in a Cu4O4 cubane-like system from truncated CI calculations, as a case study of polynuclear transition-metal complexes. We employ a new excitation selected configuration interaction (EXSCI) method based on the use of local orbitals. Taking advantage of the locality and then of the fact that the interactions vanish when the distance is large, the dimension of the CI is largely reduced. To the best of our knowledge these CI calculations are the largest one performed for polynuclear transition metal systems so far. The results show the presence of two leading ferromagnetic interactions between bridged Cu ions. Also the interactions between the unbridged Cu ions are ferromagnetic, but very weak, in contrast to the experimental data. The nature and amplitude of all the computed interactions are consistent with the relative orientation of the magnetic orbitals in the molecule, and correctly reproduce the susceptibility versus temperature curve. Our results indicate that it is possible to obtain similar fittings with sets of parameters representing different physical effects and put in evidence the drawbacks of the fitting based on oversimplified magnetic models. In this context, the presented computational strategy can be considered as a useful tool to help in the interpretation of the magnetic data and the validation of the magnetic interaction model in the polynuclear magnetic systems.Ministerio de Ciencia e Innovación de España-CTQ2009-0776

Monte Carlo ab initio simulation of the absorption spectrum of Na4

International audienceThe influence of the classical vibration of the nuclei on the visible absorption spectrum of the Na, cluster is investigated. The classical motion on the ground state potential enemy surface at a given temperature is obtained from a Metropolis Monte Carlo simulation. This dynamical process makes possible a theoretical determination of the widths of the structures in the absorption spectra. The absorption spectrum is integrated from ab initio CI calculations of excited states and oscillator streqth along the random tr&xory. A fuat simulation was achieved at T= 150 K using an ab initio CI determination of the ground and the excited states. The efficiency of the Metropolis process is improved with the use of an approximate representation for the ground state. Simulations are performed for several temperatures T= SO, 150, and 500 K. The simulated spectra are discussed in relationship with the experimental speetnm of KappeJ et al

Approaches to dealing with high‐sized polynuclear systems with ab initio methods

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 other effective interaction parameters, such as hopping integrals, on-site Coulomb repulsion, four-body exchange term, ... on 2D and spin-ladder cuprates, polyoxovanadates, as well as polynuclear molecular compound

Multi-scale multireference configuration interaction calculations for large systems using localized orbitals: Partition in zones

A new multireference configuration interaction method using localised orbitals is proposed, in which a molecular system is divided into regions of unequal importance. The advantage of dealing with local orbitals, i.e., the possibility to neglect long range interaction is enhanced. Indeed, while in the zone of the molecule where the important phenomena occur, the interaction cut off may be as small as necessary to get relevant results, in the most part of the system it can be taken rather large, so that results of good quality may be obtained at a lower cost. The method is tested on several systems. In one of them, the definition of the various regions is not based on topological considerations, but on the nature, σ or π, of the localised orbitals, which puts in evidence the generality of the approac

Direct generation of local orbitals for multireference treatment and subsequent uses for the calculation of the correlation energy

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 multireference problems. On the other hand, as it will be shown in the case of bond breaking, the control of the spatial location of the active orbitals may permit description of the desired physics with a smaller number of active orbitals than when starting from canonical molecular orbitals. The subsequent calculation of the dynamical correlation energy can be achieved with a lower computational effort either due to this reduction of the active space, or by truncation of the CAS to a shorter set of references. The ground- and excited-state energies are very close to the current complete active space self-consistent field ones and several examples of multireference singles and doubles calculations illustrate the interest of the procedur

Simple but Stronger UO, Double but Weaker UNMe Bonds: The Tale Told by Cp2UO and Cp2UNR

The free energies of reaction and the activation energies are calculated, with DFT (B3PW91) and small RECP (relativistic core potential) for uranium, for the reaction of Cp2UNMe and Cp2UO with MeCCMe and H3Si-Cl that yields the corresponding addition products. CAS(2,7) and DFT calculations on Cp2UO and Cp2UNMe give similar results, which validates the use of DFT calculations in these cases. The calculated results mirror the experimental reaction of [1,2,4-(CMe3)3C5H2]2UNMe with dimethylacetylene and [1,2,4-(CMe3)3C5H2]2UO with Me3SiCl. The net reactions are controlled by the change in free energy between the products and reactants, not by the activation energies, and therefore by the nature of the UO and UNMe bonds in the initial and final states. A NBO analysis indicates that the U-O interaction in Cp2UO is composed of a single U-O bond with three lone pairs of electrons localized on oxygen, leading to a polarized U-O fragment. In contrast, the U-NMe interaction in Cp2UNMe is composed of a and component and a lone pairof electrons localized on the nitrogen, resulting in a less polarized UNMe fragment, in accord with the lower electronegativity of NMe relative to O. The strongly polarized U(+)-O(-) bond is calculated to be about 70 kcal mol-1 stronger than the less polarized U=NMe bond

Size-consistent self-consistent configuration interaction from a complete active space

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Un parcours initiatique au problème à n-corps, à l’intention des physico-chimistes. Proposition pédagogique

Un exemple astucieux permet de mettre en évidence de façon très simple, quand le nombre N de particules augmente,