Multireference Coupled-Cluster Study of the Ground State and Lowest ExcitedStates of Cyclobutadiene

摘要

The electronic structure of the ground state and several low-lying excited statesof cyclobutadiene are studied using the new state-universal multireference coupled-cluster method with single and double excitations (MR-CCSD) augmented by a noniterative inclusion of the triple excitations (MR-CCSD(T)). Two possible ground state configurations are examined, namely the square and the distorted rectangular geometries, and the multireference coupled-cluster energy barrier for the interconversion between the two rectangular ground state structures is estimated to be 6.6 kcal/mol compared with the best theoretical value, 6.4 kcal/mol obtained using the highly accurate coupled-cluster method with full inclusion of the triple excitations (CCSDT). The ordering of electronic states for the square geometry is determined, with the ground state singlet being located 6.9 kcal/mol below the lowest triplet electronic state. We also examine the potential energy surface for the interconversion between the two equivalent second-order Jahn-Teller rhombic structures for the first excited singlet state.