1,730 research outputs found
Nuclear Structure Calculations with Coupled Cluster Methods from Quantum Chemistry
We present several coupled-cluster calculations of ground and excited states
of 4He and 16O employing methods from quantum chemistry. A comparison of
coupled cluster results with the results of exact diagonalization of the
hamiltonian in the same model space and other truncated shell-model
calculations shows that the quantum chemistry inspired coupled cluster
approximations provide an excellent description of ground and excited states of
nuclei, with much less computational effort than traditional large-scale
shell-model approaches. Unless truncations are made, for nuclei like 16O,
full-fledged shell-model calculations with four or more major shells are not
possible. However, these and even larger systems can be studied with the
coupled cluster methods due to the polynomial rather than factorial scaling
inherent in standard shell-model studies. This makes the coupled cluster
approaches, developed in quantum chemistry, viable methods for describing
weakly bound systems of interest for future nuclear facilities.Comment: 10 pages, Elsevier latex style, Invited contribution to INPC04
proceedings, to appear in Nuclear Physics
XH-stretching overtone transitions calculated using explicitly correlated coupled cluster methods
We have calculated XH-stretching (where X=O, C, F, Cl) fundamental and overtone transitions for three diatomics and a few small molecules using a local mode model. The potential energy curves and dipole moment functions are calculated using the recently developed explicitly correlated coupled cluster with single doubles and perturbative triples theory [CCSD_T_-F12] with the associated VXZ-F12 (where X=D, T, Q) basis sets. We find that the basis set convergence of calculated frequencies and oscillator strengths obtained with the explicitly correlated method is much more rapid than with conventional CCSD(T) and the Dunning type correlation consistent basis sets. Furthermore, CCSD(T)-F12 frequencies and oscillator strengths obtained with the VTZ-F12 and VQZ-F12 basis sets are found to be in excellent agreement with the CCSD(T) complete basis set limit. We find that comparison of CCSD(T)-F12 frequencies with experiment is less good. The inclusion of explicit correlation exposes the inherent error of the CCSD(T) method to overestimate vibrational frequencies, which is normally compensated by basis set incompleteness error. As a consequence, we suggest that conventional CCSD(T) in combination with the aug-cc-pVTZ or aug-cc-pVQZ basis sets is likely to yield calculated XH-stretching frequencies in closest agreement with experiment
Coupled cluster calculations of ground and excited states of nuclei
The standard and renormalized coupled cluster methods with singles, doubles,
and noniterative triples and their generalizations to excited states, based on
the equation of motion coupled cluster approach, are applied to the He-4 and
O-16 nuclei. A comparison of coupled cluster results with the results of the
exact diagonalization of the Hamiltonian in the same model space shows that the
quantum chemistry inspired coupled cluster approximations provide an excellent
description of ground and excited states of nuclei. The bulk of the correlation
effects is obtained at the coupled cluster singles and doubles level. Triples,
treated noniteratively, provide the virtually exact description
- …