To further reduce computational cost, a second approach was to use the ALEP description of the lipids, SEP description of all water molecules, but describe the protein with parameters from the cost-effective polarizable protein potential (CP 3 ), which consists of atomcentered charges and isotropic dipole-dipole polarizabilities of the standard aminoacids. Since there are no such parameters available for the retinal part of the RSB moiety (i.e. β-iononoe ring and atoms C 7 to C 15 ), we derived ad hoc those parameters in the following way: We first use PyFraMe to cut the N-C 15 bond and saturate the missing valence with a hydrogen atom and treat this fragment at the CAM-B3LYP 70 /loprop-6-31+G* level of theory. This approach will be labelled cp3 .