The structural properties of the Eu3+ complexes of three lower-rim functionalized calix[4]arenes have been investigated by means of molecular dynamics (MD) and free energy perturbation Monte Carlo (FEP-MC) simulations in OPLS acetonitrile. Ligand 1, having four 2,2’-bipyridine-5-methyl groups on the lower rim, complexes Eu3+ tightly with all four bipy (2,2’-bipyridine) moieties as bidentate ligands. Additionally strong support is raised for the binding of one counterion (nitrate) in the first coordination shell of Eu3+. Ligand 2, having four 2,2’-bipyridine-6-methyl groups on the lower rim, shows less strong binding of Eu3+. I, fact, only three of the four bipy moieties are coordinated to the Eu3+, which are likely to be in dynamic equilibrium. It is predicted that two nitrate ions are also coordinated and maybe one acetonitrile. These calculations support the explanation that this complex has a lower quantum yield of sensitized emission compared to system 1 . Eu3+ due to the fact that the bipy ligands interact less strongly with the Eu3+ and therefore will have a reduced "antenna" effect. The interaction between the bipy ligands and the Eu3+ determines, among others, the quantum yield of energy transfer. A factor also relevant to the quantum yield is the symmetry of the crystal field, which can be assessed by simulations but is not easily quantified. Calculations pn the complex of 3, having three 2,2’-bipyridine-5-methyl groups on the lower rim, with Eu3+ showed an artifact of the MD protocol, which was revealed by FEP-MC calculations and confirmed by some additional MD calculations. The structure of 3 . Eu3+ most likely has one monodentate and one bidentate nitrate coordinated to the Eu3+ besides the three tightly bound bipy moieties. This study shows that in general long MD simulations are needed.