On the other hand, excluding the core from the electronic structure of atoms and molecules to lighten the computational cost, is a standard procedure in many modern codes, and as old as the very first quantum chemical calculations. The core-valence separation remains nonetheless an approximation, which can work very well in some cases, but less in others, where effects like core polarizations, core-valence correlations, or even core-core Van der Waals interactions, might become significant. This is particularly true for heavy elements, such as actinides, in which the radial distribution of semi-core orbitals often extends outside the valence region and their contribution in the chemical interactions is important. The challenge is to understand when the trade-off of excluding the core in return for computational efficiency does not jeopardize the theoretical results.