Using a valence double-zeta polarization basis, full configuration–interaction (FCI) calculations are carried out on water at its equilibrium geometry and at geometries where the OH bond lengths are stretched until dissociation. At the same geometries and with the same basis set configuration interaction calculations at excitation levels up to hextuples, multireference singles doubles configuration interaction calculations, coupled cluster calculations at excitation levels up to quadruples, Mo/ller–Plesset perturbation theory calculations through order fifteen, and complete active space second-order perturbation theory calculations are also carried out. The static correlation contribution increase with increasing bond length. The calculations show that the coupled cluster approach has a remarkable ability to describe even relatively large static correlation contributions. The single reference perturbation expansion breaks down for larger OH bond length, while the multireference approach preserves the accuracy for the whole potential curve. At the equilibrium geometry, FCI calculations have also been carried out for the lowest state of 2A1, 2B1, and 2B2 symmetry of H2O+, and the results compared with state of the art correlation results for total energies and ionization potentials (IP’s). Differential energies (IP’s) are obtained more accurately than absolute (total) energies in the size extensive coupled cluster and perturbation approaches. For the nonsize extensive configuration interaction method errors are obtained of the same size for differential and absolute energies.

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.

The energies and structures of the 28-electron tetraatomic molecules, composed of the first row nonmetallic elements: N 4 (1), CN 2 O (2), BFN 2 (3), C 2 O 2 (4), B 2 F 2 (5), CBFO (6), C 2 FN (7), and BNO 2 (8) have been studied uniformly by ab initio methods including coupled-cluster theory. New estimates of the stability of the N 4 isomers, tetraazatetrahedrane and tetraazacyclobutadiene, are presented, and a new triplet NNNN openchain isomer has been established computationally. Potential energy surfaces of the nonpolar 1 and the polar 2 are compared. Three-membered cyclic C 2V fluorodiazaboririne has been found to be the most stable isomer similar to diazirinone (Chem. Phys. Lett. 1994, 227, 312). Linear triplet CO and BF dimers, OCCO, FBBF, and OCBF, are the most stable forms of 4, 5, and 6, respectively. The singlet cyanofluoromethylene, NCCF, the global energy minimum of 7, is 7 kcal/mol more stable than isomeric CNCF and 10 kcal/mol lower in energy than 3-fluoroazacycloprop-2-ylidene. The singlet and triplet forms of nitrosoboroxide, OBNO, the most stable isomers of 8, were found to have similar energies, within 1 kcal/mol, and the isomeric triplet OBON lies only 4 kcal/mol above OBNO. Singlet-triplet energy separations and dissociation energies to diatomic fragments are compared for the series of linear 14-and 14-electron (NNNN, NNCO, OCCO, FBBF, and OCBF) and open-chain 13-and 15-electron (NCNO, OBCF, NCCF, and OBNO) dimers. Trends in the chemical bonding in the series of 28-electron tetraatomic molecules, 1-8, stability, and possible synthetic routes, are discussed.