Multireference coupled-cluster calculations on the energy of activation in the automerization of cyclobutadiene: Assessment of the state-specific multireference Brillouin–Wigner theory

Abstract

Hilbert-space state-universal multireference coupled-cluster (MR CC) data on cyclobutadiene [A. Balková and R. J. Bartlett, J. Chem. Phys. 101, 8972 (1994)] were used as a benchmark for testing our recently developed state-specific (single-root) multireference Brillouin-Wigner coupled-cluster (MR BWCC) theory. For the energy of activation in the automerization of cyclobutadiene (i.e., the energy difference between the square and rectangular structures) at the CCSD/[3s2p1d/2s] level of theory, our MR BWCCSD method gives the value of 6.2 kcal/mol, compared to 6.5 kcal/mol given by MR CCSD. With the cc-pVDZ and cc-pVTZ/cc-pVDZ basis sets, the MR BWCCSD activation barrier is 6.4 and 7.0 kcal/mol, respectively. The effect of the triple excitations [in MR CCSD(T)] and of the frozen core approximation were estimated previously to be below 0.1 kcal/mol and in the opposite direction. This shows the way of how to arrive at a more accurate automerization barrier in future calculations: extension of the basis set seems to be more important than going beyond the CCSD(T) or MB BWCCSD level of the theory.