A Local Complete Active Space Second-Order Perturbation Theory Method

In theoretical chemistry, local methods aim at reducing the computational costs of electron correlation methods with the minimal loss in accuracy possible. Of particular relevance is the work of Pulay with Projected Atomic Orbitals, of Werner and Schütz in the development of the first linear scaling electron correlation methods, and of Neese with Pair Natural Orbitals. Although local methods were widely used to reduce the computational costs of single-reference methods, local variants of multireference methods were barely explored. The latter are computationally more demanding than the former, and exploring the effects of local approximations can become so much more meaningful. In this work we present the development and implementation of a local linear scaling variant of the CASPT2 method. Like the parent canonical method, local CASPT2 can be used to calculate reaction and excitation energies, for which we show that no significant loss in accuracy takes place. Because of the reduction of the computational costs, we extended until now the applicability of CASPT2 to more than 230 atoms and 4175 basis functions.