The influence of antisymmetrization (exchange repulsion) on the shape of internal rotation potential surfaces is investigated by comparing potential curves with and without antisymmetrization. The ab initio 6-311G(3df,2p) and 6-311G++(3df,2p) calculations show that exchange interaction, for the three ethane-family molecules (ethane, disilane, and methanol) investigated, increases the torsional amplitude and level splittings. An outcome of the exchange repulsion-triggered lowered torsional frequencies is increase in excited torsional level populations with consequent effect on thermodynamic and spectroscopic properties. <(C)> 2001 American Institute of Physics.