Despite of the large flexibility and applicability of the standard SA-CASSCF method, considered a non-black-box method, in some situations the user control over the orbitals convergence is quite limited. For instance, in larger molecules often only a hand full of active orbitals can be included in the calculations, due to computational limitations. Such a small active space bias the orbitals convergence towards the ones that optimize states, and/or configurations, with the smallest dynamic correlation energies. Good examples are the isolated transition metals that, regardless of the ground state, have a tendency to converge to the state with the lowest d -occupancy configurations, that is, 4s 2 3d n , when a single reference wave function or single d -shell (inner-valence) active space is used. A possibility to increase control over orbital convergence, as we have previously proposed, is to divide the orbitals in groups, to be optimized in different steps. Although this division may be arbitrary, we propose it to be between metal centers and ligands. The flowchart in Figure summarizes the proposed optimization. It starts with the division of the atoms between metal centers and ligands. Then the isolated metal orbitals are optimized, as shown in box I. A single set of d -orbitals is optimized in a way that it is able to describe two distinct d -occupancies with similar accuracy. The metal basis set is then tailored by removing the optimized virtual orbitals. This is made to prevent the d -orbital size to be altered during the following steps, when the ligands molecular orbitals will be optimized. The isolated ligands, that is, without metal atoms, may have their orbitals pre-optimized as well, and then merged with the tailored metal ones, as described in the box II. Although in some cases this merged initial guess may improve the molecular orbitals convergence, other automatic initial guesses can be employed. The molecular orbitals are optimized, but with the restriction that the size of the metal orbitals are kept as previously optimized because metal virtual orbitals are absent. This set of molecular orbitals, optimized with restrictions, is then projected onto the complete basis set of the metal, that is, the untailored one, to recover the complete virtual space. Finally, these orbitals are used to form a CASCI reference wave function for post-CASCI calculations, as shown in box III. Once this optimization fixes the orbitals centered at the metal and, consequently, part of the spawned reference space, independently of the system geometry, it was named partially fixed reference space (PFRS). The main difference, and advantage, of our proposed approach in comparison with the usual SA-CASSCF is the need to define two weights for the states averaging. The first weight, denoted by m, is associated to the steps in box I of Figure and can be used to control the weight of different states of the metal center when optimizing its orbitals. Since the electronic states of the isolated metals have well-defined d-occupancy, in this step the user can optimize the metal orbitals, in particular its d-orbitals, in accordance to the d-configurations that are relevant to the system/process to be described. The second weight, denoted by w and associated to the CASSCF step in box III, can be used to control the optimization of the ligands in the presence of the (already pre-optimized) metals. The removal of metal virtual orbitals in this step prevents the size change of d-orbitals, and they stay as coming from the initial optimization. Such a division enables one to study the importance of the weight for each group of orbitals for the process of interest. On the other hand, it increases the dependence of the user choice and, consequently, may result in unreliable values if the weights are not well chosen. An in depth discussion regarding the development, properties and limitations of this approach can be found in reference.