Structure and energetics of small helium clusters: Quantum simulations using a recent perturbational pair potential Small helium ( 4 He) clusters containing the lighter isotope 3 He are studied by means of quantum Monte Carlo methods. Accurate ground state energies and structural properties are obtained using accurate trial wave functions and the Tang-Tonnies-Yiu ͑TTY͒ helium-helium pair potential. The dimer 4 He-3 He is not bound; as well as the trimer 4 He 3 He 2 . The smallest cluster containing 3 He is 4 He 2 3 He with a nonrigid structure having a marked linear contribution. Interestingly, this weakly bound system, with an energy one order of magnitude less than the 4 He 3 trimer, is able to bind another 3 He atom, forming the tetramer 4 He 2 3 He 2 , which shows the odd feature of having five out of six unbound pairs. In general, the substitution of a single 4 He atom in a pure cluster with a 3 He atom leads to an energetic destabilization, as the pair 4 He-3 He is not bound. The isotopic impurity is found to perturb only weakly the distributions of the remaining 4 He atoms, which retain the high floppiness already found in the pure clusters. As the number of atoms increases the isotopic impurity has the marked tendency to stay on the surface of the cluster. This behavior is consistent with the formation of the so-called ''Andreev states'' of a single 3 He in liquid 4 He helium and droplets, where the impurity tends to form single-particle states on the surface of the pure 4 He.