For ab initio MP2 calculations within the CSM framework three alternatives can be found in the literature . The first approach, often referred to as PTE, performs a normal MP2 energy calculation on the solvated HF wave function. The response of the solvent, also called reaction field, is still on the HF level. It is the only of the three approaches that is formally consistent in the sense of second-order perturbation theory [152,153]. In the so-called PTD approach the vacuum MP2 density is used to calculate the reaction field. The third approach, often called PTED, is iterative so that the reaction field reflects the density of the first-order wave function. In contrast to the PTE approach the reaction field, i.e. the screening charges, change during the iterations until self consistency is reached. Gradients are available on the formally consistent PTE level only .