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

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 equivale...

[1]  S. Peyerimhoff,et al.  ab initio Study on the Stability and Geometry of Cyclobutadiene , 1968 .

[2]  P. Taylor,et al.  A full CI treatment of the 1A1-3B1 separation in methylene , 1986 .

[3]  Rodney J. Bartlett,et al.  Hilbert space multireference coupled-cluster methods. I: The single and double excitation model , 1991 .

[4]  Kensuke Nakamura,et al.  Second-order Jahn-Teller effect of cyclobutadiene in low-lying states. An MCSCF study , 1989 .

[5]  S. Pal,et al.  Use of Cluster Expansion Methods in the Open-Shell Correlation Problem , 1989 .

[6]  Henry F. Schaefer,et al.  A new implementation of the full CCSDT model for molecular electronic structure , 1988 .

[7]  R. Bartlett,et al.  A coupled cluster approach with triple excitations , 1984 .

[8]  L. J. Schaad,et al.  Use of molecular symmetry in coupled‐cluster theory , 1987 .

[9]  J. Paldus,et al.  Cluster relations for multireference coupled‐cluster theories: A model study , 1991 .

[10]  A. Voter,et al.  The generalized resonating valence bond description of cyclobutadiene , 1986 .

[11]  R. Bartlett,et al.  The description of N2 and F2 potential energy surfaces using multireference coupled cluster theory , 1987 .

[12]  Lionel Salem,et al.  Die elektronischen Eigenschaften von Diradikalen , 1972 .

[13]  B. Brandow.  Linked-Cluster Expansions for the Nuclear Many-Body Problem , 1967 .

[14]  M. Head‐Gordon,et al.  A fifth-order perturbation comparison of electron correlation theories , 1989 .

[15]  J. Hinze,et al.  The Unitary group for the evaluation of electronic energy matrix elements , 1981 .

[16]  R. Bartlett,et al.  The general model space effective Hamiltonian in order‐for‐order expansion , 1989 .

[17]  H. Monkhorst,et al.  Coupled-cluster method for multideterminantal reference states , 1981 .

[18]  R. Bartlett,et al.  A general model-space coupled-cluster method using a Hilbert-space approach , 1990 .

[19]  L. J. Schaad,et al.  Variational calculations on the Ag vibrational states, the automerization, and the predicted Raman spectrum of cyclobutadiene , 1990 .

[20]  U. Kaldor,et al.  Degeneracy breaking in the Hilbert‐space coupled cluster method , 1993 .

[21]  Mark R. Hoffmann,et al.  A unitary multiconfigurational coupled‐cluster method: Theory and applications , 1988 .

[22]  R. Bartlett,et al.  Coupled-cluster method for open-shell singlet states , 1992 .

[23]  S. J. Cole,et al.  Towards a full CCSDT model for electron correlation , 1985 .

[24]  B. Carpenter.  Heavy-atom tunneling as the dominant pathway in a solution-phase reaction? Bond shift in antiaromatic annulenes , 1983 .

[25]  G. Maier.  Ungewöhnliche Moleküle. Wechselspiel zwischen Theorie und Experiment , 1991 .

[26]  L. C. Snyder.  A SIMPLE MOLECULAR ORBITAL STUDY OF AROMATIC MOLECULES AND IONS HAVING ORBITALLY DEGENERATE GROUND STATES , 1962 .

[27]  L. T. Redmon,et al.  Accurate binding energies of diborane, borane carbonyl, and borazane determined by many-body perturbation theory , 1979 .

[28]  Rodney J. Bartlett,et al.  Hilbert space multireference coupled-cluster methods. II: A model study on H8 , 1992 .

[29]  F. Fratev,et al.  Ab initio study of cyclobutadiene in excited states: optimized geometries, electronic transitions and aromaticities , 1982 .

[30]  V. Staemmler,et al.  A theoretical study of the structure of cyclobutadiene , 1977 .

[31]  R. Bartlett,et al.  Ab initio calculations on the energy of activation and tunneling in the automerization of cyclobutadiene , 1988 .

[32]  Alistair P. Rendell,et al.  Triple and quadruple excitation contributions to the binding in Be clusters: Calibration calculations on Be3 , 1990 .

[33]  K. Brueckner,et al.  Many-Body Problem for Strongly Interacting Particles. II. Linked Cluster Expansion , 1955 .

[34]  R. Bartlett,et al.  A full coupled‐cluster singles and doubles model: The inclusion of disconnected triples , 1982 .

[35]  S. Sander,et al.  Fourier transform infrared spectroscopy of the NO3 nu-2 and nu-3 bands - Absolute line strength measurements , 1987 .

[36]  Romuald Lenczewski,et al.  Symmetries in Science II , 1986, Springer US.

[37]  J. Cizek.  On the Correlation Problem in Atomic and Molecular Systems. Calculation of Wavefunction Components in Ursell-Type Expansion Using Quantum-Field Theoretical Methods , 1966 .

[38]  Nicholas C. Handy,et al.  Size-consistent Brueckner theory limited to double substitutions , 1989 .

[39]  P. Löwdin,et al.  New Horizons of Quantum Chemistry , 1983 .

[40]  Klaus Ruedenberg,et al.  Electronic rearrangements during chemical reactions. II. Planar dissociation of ethylene , 1979 .

[41]  U. Kaldor,et al.  The shifted scheme in the general-model-space diagrammatic perturbation theory , 1981 .

[42]  Leszek Meissner,et al.  A coupled‐cluster method for quasidegenerate states , 1988 .

[43]  Weissberger.  Physical methods of chemistry , 1971 .

[44]  R. Bartlett,et al.  The coupled‐cluster single, double, triple, and quadruple excitation method , 1992 .

[45]  M. Newton,et al.  Potential energy surfaces of cyclobutadiene: ab initio SCF and CI calculations for the low-lying singlet and triplet states , 1978 .

[46]  R. Bartlett,et al.  A coupled‐cluster study of the ground state of C+3 , 1991 .

[47]  J. G. Radziszewski,et al.  13C NMR and polarized IR spectra of vicinally labeled [13C2]cyclobutadiene in an argon matrix: Interconversion of valence tautomers , 1988 .

[48]  J. Michl,et al.  Electronic states of cyclobutadiene heteroanalogs. Critical biradicaloids , 1989 .

[49]  M. Platz.  Kinetics and spectroscopy of carbenes and biradicals , 1990 .