The quantum-mechanical calculation of the molecular force field in rectilinear and curvilinear internal coordinate representation is discussed. A method to calculate curvilinear displacements is outlined. A formula is given to transfer the rectilinear cubic force field between isotopically related molecules. The quadratic CNDO/2 force field for CHF₃ is presented in curvilinear and rectilinear coordinate representation in order to illustrate the influence of isotopic substitution and coordinate stepsize.