Medium-mass nuclei from chiral nucleon-nucleon interactions

We compute the binding energies, radii, and densities for selected medium-mass nuclei within coupled-cluster theory and employ the "bare" chiral nucleon-nucleon interaction at order N3LO. We find rather well-converged results in model spaces consisting of 15 oscillator shells, and the doubly magic nuclei 40Ca, 48Ca, and the exotic 48Ni are underbound by about 1 MeV per nucleon within the CCSD approximation. The binding-energy difference between the mirror nuclei 48Ca and 48Ni is close to theoretical mass table evaluations. Our computation of the one-body density matrices and the corresponding natural orbitals and occupation numbers provides a first step to a microscopic foundation of the nuclear shell model.Comment: 5 pages, 5 figure

Complex coupled-cluster approach to an ab-initio description of open quantum systems

We develop ab-initio coupled-cluster theory to describe resonant and weakly bound states along the neutron drip line. We compute the ground states of the helium chain 3-10He within coupled-cluster theory in singles and doubles (CCSD) approximation. We employ a spherical Gamow-Hartree-Fock basis generated from the low-momentum N3LO nucleon-nucleon interaction. This basis treats bound, resonant, and continuum states on equal footing, and is therefore optimal for the description of properties of drip line nuclei where continuum features play an essential role. Within this formalism, we present an ab-initio calculation of energies and decay widths of unstable nuclei starting from realistic interactions.Comment: 4 pages, revtex

Computation of spectroscopic factors with the coupled-cluster method

We present a calculation of spectroscopic factors within coupled-cluster theory. Our derivation of algebraic equations for the one-body overlap functions are based on coupled-cluster equation-of-motion solutions for the ground and excited states of the doubly magic nucleus with mass number $A$ and the odd-mass neighbor with mass $A-1$. As a proof-of-principle calculation, we consider $^{16}$O and the odd neighbors $^{15}$O and $^{15}$N, and compute the spectroscopic factor for nucleon removal from $^{16}$O. We employ a renormalized low-momentum interaction of the $V_{\mathrm{low-}k}$ type derived from a chiral interaction at next-to-next-to-next-to-leading order. We study the sensitivity of our results by variation of the momentum cutoff, and then discuss the treatment of the center of mass.Comment: 8 pages, 6 figures, 3 table

Comment on "Ab Initio study of 40-Ca with an importance-truncated no-core shell model"

In a recent Letter [Phys. Rev. Lett. 99, 092501 (2007)], Roth and Navratil present an importance-truncation scheme for the no-core shell model. The authors claim that their truncation scheme leads to converged results for the ground state of 40-Ca. We believe that this conclusion cannot be drawn from the results presented in the Letter. Furthermore, the claimed convergence is at variance with expectations of many-body theory. In particular, coupled-cluster calculations indicate that a significant fraction of the correlation energy is missing.Comment: 1 page, comment on arXiv:0705.4069 (PRL 99, 092501 (2007)

Solution of large scale nuclear structure problems by wave function factorization

Low-lying shell model states may be approximated accurately by a sum over products of proton and neutron states. The optimal factors are determined by a variational principle and result from the solution of rather low-dimensional eigenvalue problems. Application of this method to sd-shell nuclei, pf-shell nuclei, and to no-core shell model problems shows that very accurate approximations to the exact solutions may be obtained. Their energies, quantum numbers and overlaps with exact eigenstates converge exponentially fast as the number of retained factors is increased.Comment: 12 pages, 12 figures (from 15 eps files) include

Ab-initio computation of neutron-rich oxygen isotopes

We compute the binding energy of neutron-rich oxygen isotopes and employ the coupled-cluster method and chiral nucleon-nucleon interactions at next-to-next-to-next-to-leading order with two different cutoffs. We obtain rather well-converged results in model spaces consisting of up to 21 oscillator shells. For interactions with a momentum cutoff of 500 MeV, we find that 28O is stable with respect to 24O, while calculations with a momentum cutoff of 600 MeV result in a slightly unbound 28O. The theoretical error estimates due to the omission of the three-nucleon forces and the truncation of excitations beyond three-particle-three-hole clusters indicate that the stability of 28O cannot be ruled out from ab-initio calculations, and that three-nucleon forces and continuum effects play the dominant role in deciding this question.Comment: 5 pages + eps, 3 figure