Heisenberg Behavior of Some Carbon-Beryllium Compounds: How Well Truncated-CI Approaches Work

This works tries to establish the performance of truncated CI calculations on the evaluation of magnetic coupling parameters with respect to available FCI estimates on a set of carbon-beryllium clusters. First-, second- and third-neighbor magnetic coupling constants have been evaluated and many body effective parameters as the cyclic terms. They result from the fitting of the low-lying states to the eigenvalues of an extended Heisenberg Hamiltonian, involving not only two-body isotropic terms but also cyclic terms. SDCI and DDCI calculations have been carried out and their performance compared with FCI ones. The impact of the basis set choice and size-consistency errors have been explored. (C) 2010 Wiley Periodicals, Inc. J Comput Chem 32: 315-324, 2011