TETRAMETHYLENE OPTIMIZED BY MRCI AND BY CASSCF WITH A MULTIPLY POLARIZED BASIS-SET

Six stationary points of tetramethylene were optimized with a 44CAS-MC SCF wave function and a basis set consisting of [4s3p2d1f] on C and [2 s1p] on H. Energies were corrected by multireference CI with all singl e and double excitations (CISD). Three stationary points were also opt imized 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 flatte r potential energy surface than previous calculations. The results sug gest that several stationary points previously located at lower levels of theory effectively coalesce into a broad plateau from which both c yclization and fragmentation are accessible along paths of monotonical ly decreasing energy. The dynamical significance of the plateau region is discussed. On the whole the results support the use of single-poin t CISD energy corrections at CASSCF optimized geometries of tetramethy lene.