EN
On the basis of the deformable body model and harmonic potential approximation, a nonlinear Hamiltonian describing rovibrational states of diatomic molecules has been derived. The proposed approach is extended to include the Simons-Parr-Finlan potential, and the obtained equations are applied in evaluation of molecular constants and in reproduction of rovibrational spectra of the ^{1}Σ state of ^{12}C^{32}S, ^{13}C^{16}O and rotational spectra of ^{12}C^{16}O molecules. A comparison with the standard Dunham results is also made.