A novel partition function for one isolated internal rotation degree of freedom is presented. Our partition function is designed for torsion potentials with one antiperiplanar and two isoenergetic synclinal (gauche) minima, as in 1,2-dichloroethane or butane. Calculating thermodynamic functions (U, S, C) for the internal rotation in a number of small carbon and silicon compounds, we compare the results to those obtained with a symmetrical internal rotation partition function. The relative energy of the conformers affects the heat capacity most strongly, and gives an additive increment to the internal energy at high temperatures. (C) 2002 Elsevier Science B.V. All rights reserved.