Topic
Coupled cluster
About: Coupled cluster is a research topic. Over the lifetime, 6280 publications have been published within this topic receiving 301055 citations.
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TL;DR: For the initially formed C3H2 collision complexes of molecular beam experiments ab initio calculations are presented in this paper, where the authors identify two reaction channels for the carbon-hydrogen exchange and explain astronomical observations of a higher c-C3H to 1-C 3H ratio in dark clouds as compared to hotter envelopes of carbon stars.
Abstract: For the initially formed C3H2 collision complexes of molecular beam experiments ab initio calculations are presented. Resolving energetics and properties of these intermediates is essential for the understanding of the reaction of C(3P) with C2H2 to form interstellar cyclic and linear isomers of C3H. Computed reaction energies agree with results from molecular beam experiments. The combination of crossed molecular beam experiments and ab initio calculations allows us to identify two reaction channels for the carbon–hydrogen exchange and to explain astronomical observations of a higher c-C3H to 1-C3H ratio in dark clouds as compared to hotter envelopes of carbon stars.
114 citations
TL;DR: In this article, two classes of non-iterative coupled-cluster (CC) methods derived from the method of moments of CC equations (MMCC) are discussed, which combine the MMCC formalism with a simplified form of multi-reference perturbation theory.
Abstract: Two recently proposed classes of non-iterative coupled-cluster (CC) methods derived from the method of moments of CC equations (MMCC) are discussed. The first approach, termed MMCC/PT, combines the MMCC formalism with a simplified form of multi-reference perturbation theory. The second approach, which leads to completely renormalized (CR) CC methods employing the left eigenstates of the similarity-transformed Hamiltonian, such as CR-CCSD , exploits the recently developed biorthogonal formulation of the MMCC theory. Both approaches are capable of improving the results of standard CC and equation-of-motion CC (EOMCC) calculations for ground-state potential energy surfaces along bond breaking coordinates and excited states dominated by two-electron transitions with computer costs similar to those characterizing the popular (and failing) CCSD(T) approximation. The performance of the basic MMCC/PT and CR-CCSD approximations, in which non-iterative corrections due to triple excitations are added to the ground-s...
114 citations
TL;DR: An iterative version of the qubit coupled cluster (QCC) method to find ground electronic energies of molecules on noisy intermediate-scale quantum (NISQ) devices is proposed and an algorithm for constructing this set that scales linearly with the size of the Hamiltonian is reported.
Abstract: An iterative version of the qubit coupled cluster (QCC) method [I. G. Ryabinkin et al., J. Chem. Theory Comput. 2019, 14, 6317] is proposed. The new method seeks to find ground electronic energies ...
114 citations
TL;DR: In the case of Ne, HF, and C(2), the alternative coupled cluster approaches yield almost identical bond length, harmonic vibrational frequency, and anharmonic constant, which are more accurate than those from traditional coupled cluster theory.
Abstract: We report a general implementation of alternative formulations of single-reference coupled cluster theory (extended, unitary, and variational) with arbitrary-order truncation of the cluster operator. These methods are applied to compute the energy of Ne and the equilibrium properties of HF and C(2). Potential energy curves for the dissociation of HF and the BeH(2) model computed with the extended, variational, and unitary coupled cluster approaches are compared to those obtained from the multireference coupled cluster approach of Mukherjee et al. [J. Chem. Phys. 110, 6171 (1999)] and the internally contracted multireference coupled cluster approach [F. A. Evangelista and J. Gauss, J. Chem. Phys. 134, 114102 (2011)]. In the case of Ne, HF, and C(2), the alternative coupled cluster approaches yield almost identical bond length, harmonic vibrational frequency, and anharmonic constant, which are more accurate than those from traditional coupled cluster theory. For potential energy curves, the alternative coupled cluster methods are found to be more accurate than traditional coupled cluster theory, but are three to ten times less accurate than multireference coupled cluster approaches. The most challenging benchmark, the BeH(2) model, highlights the strong dependence of the alternative coupled cluster theories on the choice of the Fermi vacuum. When evaluated by the accuracy to cost ratio, the alternative coupled cluster methods are not competitive with respect to traditional CC theory, in other words, the simplest theory is found to be the most effective one.
113 citations
TL;DR: The frozen pair coupled cluster approach is comparable in cost to traditional closed-shell coupled cluster methods with results that are competitive for weakly correlated systems and often superior for the description of strongly correlated systems.
Abstract: Doubly occupied configuration interaction (DOCI) with optimized orbitals often accurately describes strong correlations while working in a Hilbert space much smaller than that needed for full configuration interaction. However, the scaling of such calculations remains combinatorial with system size. Pair coupled cluster doubles (pCCD) is very successful in reproducing DOCI energetically, but can do so with low polynomial scaling (N(3), disregarding the two-electron integral transformation from atomic to molecular orbitals). We show here several examples illustrating the success of pCCD in reproducing both the DOCI energy and wave function and show how this success frequently comes about. What DOCI and pCCD lack are an effective treatment of dynamic correlations, which we here add by including higher-seniority cluster amplitudes which are excluded from pCCD. This frozen pair coupled cluster approach is comparable in cost to traditional closed-shell coupled cluster methods with results that are competitive for weakly correlated systems and often superior for the description of strongly correlated systems.
113 citations