The alteration of heat capacity or thermal expansion coefficient of chalcogenide glasses at the glass transition temperature was calculated in the frame of the Narayanaswamy-Moynihan relaxation model. The calculation was modified. The viscosity activation free energy was used in the relaxation time expression instead of activation enthalpy. This approach leads to a value of the pre-exponential factor in the relaxation time expression of about 10^-13-10^-14 s, for various chalcogenide glasses, that is in accordance with atomic vibration frequency. These values are independent of the glass composition; they are nearly the same for glasses which belong to 'strong' or 'fragile' groups. Therefore the pre-exponential factor can be excluded from the adjustable parameters in the Moynihan approach and it becomes possible to calculate the activation energy of viscosity, and viscosity near T(g), from the thermal expansion coefficient data or heat capacity data. Such calculation was carried out for As₂Te₃ which has a high crystallization tendency prohibiting the measurement of viscosity in the metastable liquid state.