Elementary isotope effects (partition functions ratios) were investigated for sensitivity to the potential energy curve shape. Three molecules H-2,H-2(+) and HF were modeled with the harmonic, Morse and Wei-Hua potentials. The decisive factors were found to be the force constants and quantumness of the system measured as omega*beta (the angular frequency times the inverse temperature). The most important change is from the harmonic to the Morse potential function and more subtle change happens if the Wei-Hua potential function is used. The isotope effects were calculated with the combination of thermodynamic integration and the path integral Monte Carlo method and compared with the standard harmonic approximation. (C) 2015 Elsevier B.V. All rights reserved.