We present an explicit expression of the vibrational partition function for the improved Manning-Rosen potential energy model. We give analytical expressions for the vibrational mean energy, vibrational specific heat, vibrational free energy, and vibrational entropy for diatomic molecules. The properties of these thermodynamic functions for the a(3)Sigma(+)(u) state of the Li-7(2) molecule are discussed in detail. (C) 2016 Published by Elsevier B.V.