The goal of this paper is to examine the performance of the conventional and renormalized single-reference coupled-cluster (CC) methods in calculations of the potential energy surface of the water molecule. A comparison with the results of the 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 CR-CCSD(T), termed CR-CC(2,3), provide substantial 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 corrected for the effect of quadruply excited clusters through 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 energies connecting the region of the global minimum with the asymptotic regions.

%B Theoretical Chemistry Accounts: Theory, Computation, and Modeling (THEORETICA CHIMICA ACTA) %V 120 %P 59-78 %G eng %N 1-3 %0 Journal Article %J The Journal of Chemical Physics %D 2007 %T Active-Space Symmetry-Adapted-Cluster Configuration-Interaction and Equation-of-Motion Cluster Methods for High Accuracy Calculations of Potential Energy Surfaces of Radicals %A Y. Ohtsuka %A Piotr Piecuch %A J. R Gour %A M. Ehara %A H. Nakatsuji %K configuration interactions %K coupled cluster calculations %K electron attachment %K excited states %K free radicals %K ground states %K ionisation %K organic compounds %K oxygen compounds %K potential energy surfaces %XThe electron-attached (EA) and ionized (IP) symmetry-adapted-cluster configuration-interaction (SAC-CI) methods and their equation-of-motion coupled-cluster (EOMCC) analogs provide an elegant framework for studying open-shell systems. As shown in this study, these schemes require the presence of higher-order excitations, such as the four-particle-three-hole (4p-3h) or four-hole–three-particle (4h-3p) terms, in the electron attaching or ionizing operator R in order to produce accurate ground- and excited-state potential energy surfaces of radicals along bond breaking coordinates. The full inclusion of the 4p-3h/4h-3p excitations in the EA/IP SAC-CI and EOMCC methods leads to schemes which are far too expensive for calculations involving larger radicals and realistic basis sets. In order to reduce the large costs of such schemes without sacrificing accuracy, the active-space EA/IP EOMCC methodology [ J. R. Gour et al., J. Chem. Phys. 123, 134113 (2005) ] is extended to the EA/IP SAC-CI approaches with 4p-3h/4h-3p excitations. The resulting methods, which use a physically motivated set of active orbitals to pick out the most important 3p-2h/3h-2p and 4p-3h/4h-3p excitations, represent practical computational approaches for high-accuracy calculations of potential energy surfaces of radicals. To illustrate the potential offered by the active-space EA/IP SAC-CI approaches with up to 4p-3h/4h-3p excitations, the results of benchmark calculations for the potential energy surfaces of the low-lying doublet states of CH and OH are presented and compared with other SAC-CI and EOMCC methods, and full CI results.

%B The Journal of Chemical Physics %V 126 %P 28 pages %8 04/2007 %G eng %N 16 %0 Journal Article %J European Physics Journal A %D 2006 %T Ab Initio Coupled-Cluster Calculations for Nuclei Using Methods of Quantum Chemistry %A Piotr Piecuch %A M. Wloch %A D. J. Dean %A J. R Gour %A M. {Hjorth-Jensen} %A T. Papenbrock %A K. Kowalski %XWe report preliminary large scale ab initio calculations of ground and excited states of 16O using quantum chemistry inspired coupled cluster methods and realistic two-body interactions. By using the renormalized Hamiltonians obtained with a no-core G-matrix approach, we obtain the virtually converged results at the level of two-body interactions. Due to the polynomial scaling with the system size that characterizes coupled cluster methods, we can probe large model spaces with up to seven major oscillator shells, for which standard non-truncated shell-model calculations are not possible.

%B European Physics Journal A %V 25 %P 485-488 %8 09/2005 %G eng %N 1 %0 Journal Article %J Physical Review Letters %D 2005 %T Ab Initio Coupled-Cluster Study of 16O %A M. Włoch %A D. J. Dean %A J. R Gour %A M. {Hjorth-Jensen} %A K. Kowalski %A T. Papenbrock %A P. Piecuch %XWe report converged results for the ground and excited states and matter density of 16O using realistic two-body nucleon-nucleon interactions and coupled-cluster methods and algorithms developed in quantum chemistry. Most of the binding is obtained with the coupled-cluster singles and doubles approach. Additional binding due to three-body clusters (triples) is minimal. The coupled-cluster method with singles and doubles provides a good description of the matter density, charge radius, charge form factor, and excited states of a one-particle, one-hole nature, but it cannot describe the first-excited 0+ state. Incorporation of triples has no effect on the latter finding.

%B Physical Review Letters %V 94 %P 212501 %8 06/2005 %G eng %N 21 %0 Journal Article %J Physical Review Letters %D 2005 %T Ab Initio Coupled-Cluster Study of O %A Piotr Piecuch %A M. Wloch %A D. J. Dean %A J. R Gour %A M. {Hjorth-Jensen} %A K. Kowalski %A T. Papenbrock %XWe report converged results for the ground and excited states and matter density of 16O using realistic two-body nucleon-nucleon interactions and coupled-cluster methods and algorithms developed in quantum chemistry. Most of the binding is obtained with the coupled-cluster singles and doubles approach. Additional binding due to three-body clusters (triples) is minimal. The coupled-cluster method with singles and doubles provides a good description of the matter density, charge radius, charge form factor, and excited states of a one-particle, one-hole nature, but it cannot describe the first-excited 0+ state. Incorporation of triples has no effect on the latter finding.

%B Physical Review Letters %V 94 %8 06/2005 %G eng %0 Journal Article %J Journal of Chemical Physics %D 2005 %T Active-Space Equation-of=Motion Coupled-Cluster Methods for Excited States of Radicals and Other Open-Shell Systems: EA-EOMCCSDt and IP-EOMCCSDt %A Piotr Piecuch %A J. R Gour %A M. Wloch %B Journal of Chemical Physics %V 123 %P 14 pages %8 10/2005 %G eng %N 13