Abstract A computational strategy to analyze Cu‐O 2 adducts based on the use of difference‐dedicated configuration interaction (DDCI) calculations is presented. The electronic structure, vertical gaps and nature of the metal–O 2 interaction, and the extension of the charge transfer between both fragments have been investigated. Relative stabilities between isomers are determined from triplet states CCSD(T) calculations. The key point of the here proposed strategy rests on the use of a rationally designed active space, containing only those orbitals, which optimize the interaction pathways between LCu and O 2 fragments. The procedure has been tested on a broad set of model and synthetic biomimetic systems, the results compared with previous theoretical evaluations and/or available experimental data. Our study indicates that this strategy can be considered as an alternative approach to multireference second‐order perturbation theory methods to deal with this type of systems with remarkable biradical nature. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011