A multireference coupled-cluster study of the ground state and lowest excited states of cyclobutadiene

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Abstract
The electronic structure of the ground state and several low‐lying excited states of 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−1 compared with the best theoretical value, 6.4 kcal mol−1 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−1 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. When comparing the MRCC energies with the results provided by various single‐ and multireference correlation methods, the critical importance of including both the dynamic and nondynamic correlation for a qualitatively correct description of the electronic structure of cyclobutadiene is emphasized. We also address the invariance properties of the present MRCC methods with respect to the alternative selections of reference orbital spaces.