Browsing by Author "Gomez, John A."

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Incorporating Spin Symmetry Projection Into Traditional Coupled Cluster Theory
    (2017-03-14) Gomez, John A.; Scuseria, Gustavo E
    In electronic structure theory, restricted single-reference coupled cluster (CC) captures weak correlation but fails catastrophically under strong correlation. Spin-projected unrestricted Hartree-Fock (SUHF), on the other hand, misses weak correlation but captures a large portion of strong correlation. The theoretical description of many important processes, e.g. molecular dissociation, requires a method capable of accurately capturing both weak- and strong correlation simultaneously, and would likely benefit from a combined CC-SUHF approach. Based on what we have recently learned about SUHF written as particle-hole excitations out of a symmetry-adapted reference determinant, we here propose a heuristic coupled cluster doubles model to attenuate the dominant spin collective channel of the quadratic terms in the coupled cluster equations. Proof of principle results presented here are encouraging and point to several paths forward for improving the method further.
  • Thumbnail Image
    Item
    Recoupling the singlet- and triplet-pairing channels in single-reference coupled cluster theory
    (AIP Publishing LLC., 2016) Gomez, John A.; Henderson, Thomas M.; Scuseria, Gustavo E.
    It is well known that single-reference coupled cluster theory truncated to low orders of excitations gives the right answer for the right reason when systems are dominated by dynamical or weak correlation. Static or strong correlation is more problematic, causing often catastrophic breakdown of restricted coupled cluster. This failure can be remedied, e.g., by allowing symmetry breaking in the reference or taking a multi-reference approach, but poses an interesting theoretical problem, especially since many groups have found that simplifying the T2 operator or the doubles amplitude equations gives better results. In singlet-paired coupled cluster, eliminating the triplet-pairing channel recovers reasonable qualitative behavior for strong correlation at the cost of a decreased description of dynamical correlation in weakly correlated situations. This behavior seems to hold for both closed- and open-shell systems. In this work, we explore the coupling of the singlet- and triplet-pairing channels of T2 and attempt to recouple them in order to recover dynamical correlation without reintroducing catastrophic failure due to strong correlation. In the weakly correlated regime, these pairing channels are only weakly coupled, and a simple recoupling gives good results. However, as strong correlation dominates, the coupling strength between the singlet- and triplet-pairing channels increases, making it difficult to perturbatively recouple the singlet- and triplet-pairing channels in this regime.
  • Thumbnail Image
    Item
    Singlet-paired coupled cluster theory for open shells
    (AIP Publishing LLC, 2016) Gomez, John A.; Henderson, Thomas M.; Scuseria, Gustavo E.; Smalley Institute for Nanoscale Science and Technology
    Restricted single-reference coupled cluster theory truncated to single and double excitations accurately describes weakly correlated systems, but often breaks down in the presence of static or strongcorrelation. Good coupled cluster energies in the presence of degeneracies can be obtained by using a symmetry-broken reference, such as unrestricted Hartree-Fock, but at the cost of good quantum numbers. A large body of work has shown that modifying the coupled cluster ansatz allows for the treatment of strong correlation within a single-reference, symmetry-adapted framework. The recently introduced singlet-paired coupled cluster doubles (CCD0) method is one such model, which recovers correct behavior for strong correlation without requiring symmetry breaking in the reference. Here, we extend singlet-paired coupled cluster for application to open shells via restricted open-shell singlet-pairedcoupled cluster singles and doubles (ROCCSD0). The ROCCSD0 approach retains the benefits of standard coupled cluster theory and recovers correct behavior for strongly correlated, open-shell systems using a spin-preserving ROHF reference.
  • Thumbnail Image
    Item
    Towards the Solution of the Many-Electron Problem in Real Materials: Equation of State of the Hydrogen Chain with State-of-the-Art Many-Body Methods
    (American Physical Society, 2017) Motta, Mario; Ceperley, David M.; Chan, Garnet Kin-Lic; Gomez, John A.; Gull, Emanuel; Guo, Sheng; Jiménez-Hoyos, Carlos A.; Lan, Tran Nguyen; Li, Jia; Ma, Fengjie; Millis, Andrew J.; Prokof’ev, Nikolay V.; Ray, Ushnish; Scuseria, Gustavo E.; Sorella, Sandro; Stoudenmire, Edwin M.; Sun, Qiming; Tupitsyn, Igor S.; White, Steven R.; Zgid, Dominika; Zhang, Shiwei
    We present numerical results for the equation of state of an infinite chain of hydrogen atoms. A variety of modern many-body methods are employed, with exhaustive cross-checks and validation. Approaches for reaching the continuous space limit and the thermodynamic limit are investigated, proposed, and tested. The detailed comparisons provide a benchmark for assessing the current state of the art in many-body computation, and for the development of new methods. The ground-state energy per atom in the linear chain is accurately determined versus bond length, with a confidence bound given on all uncertainties.