The stationary points on the potential energy surface (PES) of tetramethylene have been investigated using highly correlated multireference methods and extended basis sets. Full geometry optimization using analytic gradients as well as systematic scans of the PES employing different basis sets show that all minima and most of the saddle points found at the lower computational level (mostly CASSCF) cease to exist. Finally, only the G2 (gauche cyclization with conrotatory double CH2 twist) and the CT (cis-trans isomerization) saddle points have been confirmed. Activation and reaction enthalpies have been computed and are within 1-2 kcal/mol in agreement with experimental results. (C) 2003 American Institute of Physics.