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 International Journal of Quantum Chemistry %D 2006 %T Automated derivation and parallel computer implementation of renormalized and active-space coupled-cluster methods %A Piotr Piecuch %A So Hirata %A K. Kowalski %A P.-D. Fan %A Theresa L. Windus %XOur recent efforts that have led to an automated derivation and computer implementation of the renormalized and active-space coupled-cluster {(CC)} methods with Tensor Contraction Engine {(TCE)} are summarized. The {TCE-generated} renormalized and active-space {CC} computer codes are parallel and applicable to closed- and open-shell references, enabling accurate calculations of potential energy surfaces along bond-breaking coordinates and excited states displaying a significant multi-reference character. The effectiveness of the new codes in describing electronic quasi-degeneracies is illustrated by the renormalized {CC} calculations of the potential energy curve of {HCl} and the active-space {CC} calculations for the low-lying excited states of the Be3 system. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

%B International Journal of Quantum Chemistry %V 106 %P 79–97 %8 08/2005 %G eng %N 1 %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 The Journal of Chemical Physics %D 2005 %T Active-Space Coupled-Cluster Study of Electronic States of Be 3 %A Piotr Piecuch %A K. Kowalski %A S. Hirata %A M. Wloch %A T.L. Windus %XThe active-space coupled-cluster (CC) and equation-of-motion (EOM) CC methods with all single and double excitations with triple excitations defined via active orbitals (CCSDt, EOMCCSDt), as implemented with TENSOR CONTRACTION ENGINE, are applied to the challenging Be3 system, which is characterized by a large number of low-lying excited states dominated by two-electron transitions and significant high-order correlation effects in the ground electronic state. It is demonstrated that the CCSDt and EOMCCSDt methods provide an excellent description of complicated electronic quasidegeneracies present in the Be3 cluster. Different strategies for defining triple excitations within the CCSDt∕EOMCCSDt approach are discussed.

%B The Journal of Chemical Physics %V 123 %P 6 pages %8 08/2005 %G eng %N 7 %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 %0 Journal Article %J The 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 Jeffrey R. Gour %A Piotr Piecuch %A Marta Wloch %K coupled cluster calculations %K free radicals %K molecular electronic states %XThe previously developed active-space coupled-cluster (CC) and equation-of-motion (EOM) CC methods are extended to radicals and other open-shell systems by combining them with the electron attached (EA) and ionized (IP) EOMCC approaches. As illustrated by the calculations for the CH and OH radicals, the resulting EA-EOMCCSDt and IP-EOMCCSDt theories are capable of providing a highly accurate description of the electronic spectra of radical systems, including excited states displaying a manifestly multideterminantal nature, with the low costs that are not much greater that those characterizing the standard EOMCC singles and doubles method.

%B The Journal of Chemical Physics %V 123 %P 134113–14 %G eng %N 13