Calculation of the two-dimensional non-separable partition function for two molecular systems.

We present the application of an accurate quantum treatment, called two-dimensional non-separable (2D-NS), to the calculation of internal rotation partition functions of molecules with two rotors. This methodology involves full coupling in the kinetic and potential energies; the later is written as a Fourier series type potential. The resulting Hamiltonian is introduced in the Schrödinger equation and solved by the variational method. The method was applied to the 2-propenol and to the 3-fluoro-2-propenol molecular systems. The former molecule presents weak coupling between the torsion, whereas the later is an example of strong coupling. The comparison of 2D-NS with one-dimensional accurate models that involve separation of the two torsions, indicate that a separable model is inadequate to study systems in the strong coupling regime. The results indicate that for the case of strong coupling the multi-conformer harmonic approximation gives better results than a separable anharmonic model.