Chemical systems with a multiconfigurational electronic structure continue to represent a major challenge for modern quantum chemistry. The CASSCF/CASPT2 approach [1] is one of the most elegant ways to treat non-dynamical electron correlation; however, it is severely limited by the size of the active space, which may include at most 15-16 electrons in 15-16 active orbitals.We have recently extended this method to the restricted active space RASSCF/RASPT2 regime [2], which permits the address of significantly larger active spaces, including up to 30 electrons in 30 orbitals, by considering more limited sets of excitations.We are working at the moment on a further extension, the Split-CAS method,[3] which allows the selection of the important configurations, by still using large active spaces.In this lecture we will describe the main features of the Split-CAS method and we will show our recent applications concerning the study of polymetallic systems and organic oligomers of increasing size [4]. [1] B. O. Roos, P. R. Taylor, and P. E. M. Siegbahn, Chem. Phys. 48, 157 (1980); B. O. Roos, in Advances in Chemical Physics: Ab Initio Methods in Quantum Chemistry-II, Wiley, Chichester, England, (1987), Chap. 69, p. 399.[2] P.-A. Malmqvist, K. Pierloot., A. R. Moughal Shahi, C. J. Cramer and L. Gagliardi The Restricted Active Space followed by second order perturbation theory method: theory and application to the study of CuO2 and Cu2O2 systems J. Chem. Phys. 128, 204109 (2008)[3] G. Li Manni, F. Aquilante, and L. Gagliardi in preparation[4] A. R. Moughal Shahi, C. J. Cramer, and L. Gagliardi Second-order perturbation theory with complete and restricted active space reference functions applied to oligomeric unsaturated hydrocarbons DOI: 10.1039/b912607d (2009)

Extension of the CASPT2 and RASPT2 methods for the treatment of nondynamical electron correlation: The split-CAS method

239th National Meeting (2010)

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