Articles you may be interested inThe multi-configuration self-consistent field method within a polarizable embedded framework J. Chem. Phys. 139, 044101 (2013); 10.1063/1.4811835Approximately extensive modifications of the multireference configuration interaction method: A theoretical and practical analysis Large multiconfiguration selfconsistentfield wave functions for the ozone moleculeWe have developed a method to analyze multi-configuration self-consistent field wave functions for open-shell systems, which may consist of a very large number of Slater determinants, by calculating their expectation values for ͑local͒ one-and two-particle operators in a numerically efficient way. Although the proposed formalism is fully general and applies to any operators referring to a subset of arbitrary orthogonal molecular orbitals, the local operators we use here are defined in terms of localized molecular orbitals. This choice permits to perform a chemically intuitive analysis of the local spin, population and charge fluctuation, also allowing to study electron correlation effects in chemical bonding. We present some applications for ionic and covalent systems considering four small molecules, H 2 , N 2 , H 2 O and CH 4 , as well as a binary transition metal complex, ͓Fe 2 S 6 ͔ 6Ϫ .

For the ternary thioferrate crystal Na 6 Fe 2 S 6 , ab initio quantum chemical calculations using a cluster model ansatz have been performed to examine the magnetic coupling of the two half-filled Fe 3d shells in the isolated dimeric [Fe 2 S 6 ] 6complexes having the structure of edge-linked double tetrahedra. The activeelectron approach using complete active space configuration interaction (CASCI) with 10 electrons in 10 orbitals yields the multiplet splitting of a two-center Heisenberg Hamiltonian with an antiferromagnetic coupling constant J ) -19 cm -1 , which is by a factor of 5 smaller than the experimental value. Correlation effects are essential for the magnetic coupling, as the application of multireference second-order Møller-Plesset perturbation theory based on the CIPSI algorithm (CAS-2nd) and the recently proposed difference-dedicated CI method lead to values J(4f5) ) -158 and -66 cm -1 , respectively, which clearly agree better with experiment. The different electronic contributions to the chemical bonding in the binuclear transition metal complex have been investigated using the constrained space orbital variation method.