Six stationary points of tetramethylene were optimized with a 44CAS-MCSCF wave function and a basis set consisting of [4s3p2d1f] on C and [2s1p] on H. Energies were corrected by multireference CI with all single and double excitations (CISD). Three stationary points were also optimized with CISD from a 22CAS reference and a basis set consisting of [3s2p1d] on C and [2s] on H. The current results predict a much flatter potential energy surface than previous calculations. The results suggest that several stationary points previously located at lower levels of theory effectively coalesce into a broad plateau from which both cyclization and fragmentation are accessible along paths of monotonically decreasing energy. The dynamical significance of the plateau region is discussed. On the whole the results support the use of single-point CISD energy corrections at CASSCF optimized geometries of tetramethylene.