We present a closed-form expression of the classical vibrational partition function for the improved Rosen-Morse potential energy model. We give explicit expressions for the vibrational mean energy, vibrational specific heat, vibrational free energy, and vibrational entropy for diatomic molecule systems. The properties of these thermodynamic functions for the Na-2 dimer are discussed in detail. We find that the improved Rosen-Morse potential model is superior to the harmonic oscillator in calculating the heat capacity for the Na-2 molecules. (C) 2017 Elsevier B.V. All rights reserved.