The d orbitals size relevance in the transition metal compounds description
The theoretical study of molecules containing first-row transition metals can be challenging, especially when the process depends on configurations with distinct d orbitals occupation. The common approaches for wave-function methods used in the literature to treat these systems can become quickly infeasible with the increase in the number of atoms or in the basis set size, due to its high computational cost. In this work we explore properties of the d orbitals in complete active space wavefunctions and the impact of these orbitals optimisation over the results of multireference methods. We propose a set of four approaches that allows the user to restrict the optimisation of the molecular orbitals increasing the control over the final set. Our results shown the possibility to accurately describe the electronic structure of 3d transition metal compounds, with a smaller computational cost, using this restricted orbital optimisation.