Physical analysis of the through-ligand long-distance magnetic coupling: spin-polarization versus Anderson mechanism.

The physical factors governing the magnetic coupling between two magnetic sites are analyzed and quantified as functions of the length of the bridging conjugated ligand. Using wave-function-theory based ab initio calculations, it has been possible to separate and calculate the various contributions to the magnetic coupling, i.e. the direct exchange, the spin polarization and the kinetic exchange. It is shown in model systems that while the Anderson mechanism brings the leading contribution for short-length ligands, the spin polarization dominates the through-long-ligand couplings. Since the spin polarization decreases more slowly than the kinetic exchange, highly spin polarizable bridging ligands would generate a good magneto-communication between interacting magnetic units.

Determination of spin Hamiltonians from projected single reference configuration interaction calculations. I. Spin 1/2 systems.

The most reliable wave-function based treatments of magnetic systems usually start from a complete active space self-consistent field calculation of the magnetic electrons in the magnetic orbitals, followed by extensive and expensive configuration interaction (CI) calculations. This second step, which introduces crucial spin polarization and dynamic correlation effects, is necessary to reach reliable values of the magnetic coupling constants. The computational cost of these approaches increases exponentially with the number of unpaired electrons. The single-determinantal unrestricted density functional Kohn-Sham calculations are computationally much simpler, and may provide reasonable estimates of these quantities, but their results are strongly dependent on the chosen exchange-correlation potential. The present work, which may be seen as an ab initio transcription of the unrestricted density functional theory technique, returns to the perturbative definition of the Heisenberg Hamiltonian as an effective Hamiltonian, and proposes a direct estimate of its diagonal energies through single reference CI calculations. The differences between these diagonal terms actually determine the entire Heisenberg Hamiltonian. The reference determinants must be vectors of the model space and the components on the other vectors of the model space are cancelled along the iterative process. The method is successfully tested on a series of bicentric and multicentric spin 12 systems. The projected single reference difference dedicated CI treatment is both accurate and of moderate cost. It opens the way to parameter-free calculations of large spin assemblies.

FORTRAN interface for code interoperability in quantum chemistry: the Q5Cost library.

Ab initio quantum-chemistry programs produce and use large amounts of data, which are usually stored on disk in the form of binary files. A FORTRAN library, named Q5Cost, has been designed and implemented in order to allow the storage of these data sets in a special data format built with the HDF5 technology. This data format allows the data to be represented as tree structures and is portable between different platforms and operating systems, making code interoperability and communication much easier. The libraries have been used to build many interfaces among different quantum chemistry codes, and the first scientific applications have been realized. This activity was carried out within the COST in Chemistry D23 project "MetaChem", in the Working Group "A meta-laboratory for code integration in ab initio methods".

Multistate active spaces from local CAS-SCF molecular orbitals: the photodissociation of HFCO as an example.

A recently developed algorithm to generate localized molecular orbitals (LMO) is applied to the study of excited states along a photodissociation process. The LMOs allow for the selection of a consistent complete active space (CAS) for the simultaneous description of all the electronic states involved in a multistate process on the basis of simple chemical criteria. The local nature of the orbitals is used to label them in a unique way that does not depend on the molecular geometry. The selection of the electronic configurations of interest for the set of target states on only the basis of the dominant excitations required by the simplest configuration interaction (CI) descriptions for both ground and excited states is fairly simplified. The following of the changes in the nature of the states along cuts in the set of potential energy surfaces (PES) is also simpler. The C--F bond breaking and the states involved in the photodissociation of the HFCO system, together with another geometrical path along the PES of the same previous states, are studied by way of examples in this work. The local nature of the MOs warrants that this small system is representative enough to show the advantages of the procedure and that its application to larger systems (R-FCO systems) would be straightforward.