Capturing this term, which is associated with the explicit interaction of electrons beyond the meanfield approximation, requires moving beyond HF. This is done by approximating the multielectron wavefunction by more than one Slater determinant. Either by generating substituted determinants as a sum (configuration interaction, CI) or with an exponential approach (coupled cluster, CC). Furthermore, the Rayleigh-Schrödinger perturbation theory approach can be used which leads to Møller-Plesset perturbation theory of n-th order (MPn). Discussing the details of these approaches is beyond the scope of this review, especially given their negligible usage in AS-ALD. The interested reader is referred to textbooks on the topic. However, two major approaches shall be discussed which are important methods to benchmark the accuracy of DFT computations. Although they are not used in AS-ALD yet, it is known from other communities that benchmarking is important and it should be extended in AS-ALD. The most economical way to capture the correlation energy is MP2. This approach only requires orbital energies and a further set of integrals. Modern MP2 implementations combine smart approximations to these integrals (e.g., density fitting, Cholesky decomposition, Tensor network) are close to the efficiency of (hybrid) DFT computations and thus competitive for molecular approaches. Unfortunately, when using PBC, MP2 is very costly and hardly used. The second method of relevance is DLPNO-CCSD(T) and related methods. CCSD(T) stands for CC with singles, doubles and perturbative triples which refers to the number of electrons moved from occupied to virtual orbitals when creating the additional Slater determinants (eq. 2.8). While this approach is only applicable to small molecules, modern implementations with clever approximations are applicable to systems with hundreds of atoms and thousands of electrons. DLPNO stands for domain localized pair natural orbitals which summarizes the approximations used. In a nutshell, the approach makes use of the fact that electron correlation is large when electrons are close to each other and weak otherwise. Thus, the correlation energy of electrons which are far apart is computed with "cheaper" methods (MP2). Also, molecular orbitals are transformed via unitary transformations (a mathematical operation that does not change the energy) to localize them on parts of the system and thus further speed-up the computations.