State-specific Brillouin-Wigner multireference coupled cluster study of the singlet-triplet separation in the tetramethyleneethane diradical

The potential energy curves for the twisting of tetramethyleneethane in its lowest singlet and triplet states were calculated by the state-specific two-reference Brillouin-Wigner coupled-cluster method with single and double excitations. The calculated potential energy curves are essentially the same as those obtained by the two-determinant CCSD method, and they are also in agreement with the previously reported density functional theory results. Our data bring support for the previously suggested interpretation of experimental data on tetramethyleneethane in the gas phase and in the matrix.