Conventional single-reference methods fail when bond breaking and other situations characterized by larger non-dynamical correlation effects are examined. In consequence, the adequate treatment of molecular potential energy surfaces involving signiﬁcant bond rearrangements has been the domain of expert multi-reference methods. The question arises if one can develop practical single-reference procedures that could be applied to at least some of the most frequent multi-reference situations, such as single and double bond dissociations. This question is addressed in the present paper by examining the performance of the conventional and renormalized coupled-cluster (CC) methods in calculations of the potential enery surface of the water molecule. A comparison with the results of the highly accurate internally contracted multi-reference conﬁguration interaction calculations including the quasi-degenerate Davidson correction (MRCI(Q)) and the spectroscopically accurate potential energy surface of water resulting from the use of the energy switching (ES) approach indicates that the relatively inexpensive completely renormalized (CR) CC methods with singles (S), doubles (D), and a non-iterative treatment of triples (T) or triples and quadruples (TQ), such as CR-CCSD(T), CR-CCSD(TQ), and the recently developed rigorously size extensive extension of the CR-CCSD(T), termed CR-CC(2,3), provide considerable improvements in the results of conventional CCSD(T) and CCSD(TQ) calculations at larger internuclear separations. It is shown that the CR-CC(2,3) results a posteriori corrected for the effect of quadruply excited clusters (the CR-CC(2,3)+Q approach) can compete with the highly accurate MRCI(Q) data. The excellent agreement between the CR-CC(2,3)+Q and MRCI(Q) results suggests ways of improving the global potential energy surface of water resulting from the use of the ES approach in the regions of intermediate bond stretches and intermediate and higher energies connecting the region of the global minimum with the asymptotic regions. In addition to the examination of the performance of the CR-CCSD(T), CR-CCSD(TQ), CR-CC(2,3), and CR-CC(2,3)+Q approaches, we provide a thorough review of the method of moments of CC equations (MMCC), as applied to ground electronic states, including the most recent biorthogonal formulation of MMCC theory employing the left eigenstates of the similarity-transformed Hamiltonian, and other mathematical and physical concepts that lie behind all renormalized CC approximations. In particular, we discuss the similarities and differences between the older CR-CCSD(T) and CR-CCSD(TQ) approximations and the recently formulated size extensive renormalized CC methods, such as CR-CC(2,3), and open questions that emerge in the process of designing higher-order schemes based on the biorthogonalMMCC formalism, such as CR-CC(2,4), which describe the combined effect of triples (already present in CR-CC(2,3) calculations) and quadruples in a proper manner.

JF - Progress in Theoretical Chemistry and Physics VL - 16 IS - Part 1 ER - TY - JOUR T1 - Shape and Structure of N = Z 64Ge: Electromagnetic Transition Rates from the Application ￼of the Recoil Distance Method to a Knockout Reaction JF - PHYSICAL REVIEW LETTERS Y1 - 2007 A1 - K. Starosta A1 - A. Dewald A1 - A. Dunomes A1 - P. Adrich A1 - A.M. Amthor A1 - T. Baumann A1 - D. Bazin A1 - M. Bowen A1 - B. A. Brown A1 - A. Chester A1 - A. Gade A1 - D. Galaviz A1 - T. Glasmacher A1 - T. Ginter A1 - M. Hausmann A1 - M. Horoi A1 - J. Jolie A1 - B. Melon A1 - D. Miller A1 - V. Moeller A1 - R.P. Norris A1 - T. Pissulla A1 - M. Portillo A1 - W. Rother A1 - Y. Shimbara A1 - A. Stolz A1 - C. Vaman A1 - P. Voss A1 - D. Weisshaar A1 - V. Zelevinsky VL - 99 UR - http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.99.042503 ER - TY - JOUR T1 - Z = 0 Shell Gap near 100Sn from Intermediate-Energy Coulomb Excitations ￼in Even-Mass 106-112Sn Isotopes JF - PHYSICAL REVIEW LETTERS Y1 - 2007 A1 - C. Vaman A1 - C. Andreoiu A1 - D. Bazin A1 - A. Beccerril A1 - B. A. Brown A1 - C.M. Campbell A1 - A. Chester A1 - J.M. Cook A1 - D.C. Dinca A1 - A. Gade A1 - D. Galaviz A1 - T. Glasmacher A1 - M. {Hjorth-Jensen} A1 - M. Horoi A1 - D. Miller A1 - V. Moeller A1 - W. F. Mueller A1 - A. Schiller A1 - K. Starosta A1 - A. Stolz A1 - J.R. Terry A1 - A. Volya A1 - V. Zelevinsky A1 - H. Zwahlen VL - 99 UR - http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.99.162501 ER - TY - JOUR T1 - Extrapolating Potential Energy Surfaces by Scaling Electron Correlation at a Single Geometry JF - Chemical Physics Letters Y1 - 2006 A1 - A.J.C. Varandas A1 - Piotr Piecuch AB -It is shown that the molecular potential energy surface corresponding to a high level of ab initio theory can be accurately predicted by performing calculations with smaller basis sets and then scaling the electron correlation at a single point calculated with the larger target basis set.

VL - 430 IS - 4-6 ER - TY - JOUR T1 - Where Does the Planar-to-Nonplanar Turnover Occur in Small Gold Clusters JF - Journal of the American Chemical Society Y1 - 2005 A1 - Piotr Piecuch A1 - R.M. Olson A1 - S. Varganov A1 - M.S. Gordon A1 - S. Chretien A1 - H. Metiu A1 - K. Kowalski A1 - S. A. Kucharski A1 - M. Musial AB -Several levels of theory, including both Gaussian-based and plane wave density functional theory (DFT), second-order perturbation theory (MP2), and coupled cluster methods (CCSD(T)), are employed to study Au6 and Au8 clusters. All methods predict that the lowest energy isomer of Au6 is planar. For Au8, both DFT methods predict that the two lowest isomers are planar. In contrast, both MP2 and CCSD(T) predict the lowest Au8 isomers to be nonplanar.

VL - 127 IS - 3 ER -