RPA abstracts

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The Selfconsistent Quasiparticle RPA and Its Description of Thermal Pairing Properties in Nuclei

Nguyen Dinh Dang

1 - Heavy-Ion Nuclear Physics Laboratory, Nishina Center for Accelerator-Based Science RIKEN 2-1 Hirosawa, Wako city, 351-0198 Saitama, Japan

2 - Institute for Nuclear Science and Techniques, Vietnam Atomic Energy Commission Hanoi - Vietnam

The Selfconsistent Quasiparticle RPA (SCQRPA) is constructed [1] to study the effects of fluctuations on pairing properties in finite systems. The theory is applied to nuclei at finite temperature [2] and angular momentum [3]. Particle number projection is taken into account within the Lipkin-Nogami method. Several issues such as the smoothing of superfluid-normal phase transition, thermally assisted pairing in hot rotating nuclei, extraction of the nuclear pairing gap using an improved odd-even mass difference are discussed [4]. Finally, a novel approach of embedding the projected SCQRPA eigenvalues in the canonical ensemble (CE) is proposed (the CE-SCQRPA) [5]. Applied to a doubly-folded equidistant multilevel pairing model, the proposed CE-SCQRPA produces results in good agreement with those obtained by using the exact eigenvalues, whenever the latter are possible, and is workable also for large values of particle number (N>14), where the diagonalisation of the pairing Hamiltonian is impracticable.

References

[1] N. Quang Hung and N. Dinh Dang, 
Phys. Rev. C 76 (2007) 054302 and 77 (2008) 029905(E).

[2] N. Dinh Dang and N. Quang Hung, 
Phys. Rev. C 77 (2008) 064315.

[3] N. Quang Hung and N. Dinh Dang, 
Phys. Rev. C 78 (2008) 064315.

[4] N. Quang Hung and N. Dinh Dang, 
Phys. Rev. C 79 (2009) 054328.

[5] N. Quang Hung and N. Dinh Dang, in preparation

The two faces of RPA: density functional theory and many-body perturbation theory

Xavier Gonze

Unité Physico-Chimie et de Physique des Matériaux (PCPM), Université catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgique

The RPA expression for total energy might be derived either within density functional theory, (in the adiabatic-connection fluctuation-dissipation framework, by setting the exchange-correlation kernel to zero), or from many-body perturbation theory (from the Nozières functional in the GW approximation for the self-energy, when the green's function corresponds to an energy-independent one-body Hamiltonian). I will review results in which these two faces of RPA appear. First, the RPA exchange-correlation potential is obtained from a linear-response Sham-Schlüter equation [1,2]. Second, the RPA band gap (I-A expression) corresponds to non-renormalized G_0 W_0 [3]. I will also show that density functional theory within the RPA provides a correct description of bond dissociation for the hydrogen dimer in a spin-restricted Kohn-Sham formalism, i.e., without artificial symmetry breaking, with important static (left-right) correlation [4]. Although exact at infinite separation and accurate near the equilibrium bond length [5], the RPA dissociation curve displays unphysical repulsion at larger but finite bond lengths [4,6].

References

[1] Y.-M. Niquet, M. Fuchs, X. Gonze, J. Chem. Phys. 118, 9504 (2003)

[2] Y.-M. Niquet, M. Fuchs, X. Gonze, Phys. Rev. A 68, 032507 (2003)

[3] Y.-M. Niquet, X. Gonze, Phys. Rev. B 70, 245115 (2004)

[4] M. Fuchs, Y.-M. Niquet, X. Gonze, K. Burke, J. Chem. Phys. 122, 094116 (2005)

[5] M. Fuchs, X. Gonze, Phys. Rev. B 65, 235109 (2002)

[6] M. Fuchs, K. Burke, Y.-M. Niquet, X. Gonze, Phys. Rev. Lett. 90, 189701 (2003)

How approximate is the random phase approximation? Comparing RPA against full configuration-interaction calculations

Calvin W. Johnson

Department of Physics, San Diego State University, USA

I compare the random phase approximation against numerically exact configuration-interaction calculations in the nuclear shell model and find good, not perfect, results. This is of particular importance when one is using a phenomenological interaction. I also discuss the so-called "collapse" of RPA in "phase changes" and illustrate the difference between first- and second-order phase changes and how collapse occurs in only one.

References

[1] I. Stetcu and C. W. Johnson, Phys. Rev. C 66, 034301 (2002); Phys. Rev. C 67, 043315 (2003); Phys. Rev. C 69, 024311 (2004). [2]C. W. Johnson and I. Stetcu, Phys. Rev. C 80, 024320 (2009).

RPA and coupled cluster theory & some recent results including range separation

Gustavo E. Scuseria

Department of Chemistry and Department of Physics & Astronomy, Rice University, Houston, Texas, USA

The recent realization that the ground-state correlation energy of the random phase approximation (RPA) is intimately connected to an approximate coupled cluster doubles (CCD) model [1], opens interesting avenues for mixing RPA with DFT [2]. I will describe some of the recent work done in our research group on RPA, including applications to van der waals and noncovalent interactions [3], the importance of the reference state [4], a simple second-order approximation [5], and a second-order exchange (SOSX) correction that restores antisymmetry [6].

References

[1] The ground state correlation energy of the Random Phase Approximation from a ring Coupled Cluster Doubles approach, G. E. Scuseria, T. M. Henderson, and D. C. Sorensen, J. Chem. Phys. 129, 231101 (2008).

[2] Long-range-corrected hybrids including random phase approximation correlation, B. G. Janesko, T. M. Henderson, and G. E. Scuseria, J. Chem. Phys. 130, 081105 (2009).

[3] Long-range corrected hybrid functionals including random phase approximation correlation: Application to noncovalent interactions, B. G. Janesko, T. M. Henderson, and G. E. Scuseria, J. Chem. Phys. 131, 034110 (2009).

[4] The role of the reference state in long-range random phase approximation correlation, B. G. Janesko and G. E. Scuseria, J. Chem. Phys. 131, 154106 (2009).

[5] Coulomb-only second-order perturbation theory in long-range-corrected hybrid density functionals, B. G. Janesko and G. E. Scuseria, Phys. Chem. Chem. Phys. 11, 9677 (2009).

[6] Hybrid functionals including random phase approximation correlation and second-order screened exchange, J. Paier, B. G. Janesko, T. M. Henderson, G. E. Scuseria, A. Gruneis, and G. Kresse, J. Chem. Phys. submitted.

Promising first results with an RPA correlation functional based on the frequency-dependent Kohn-Sham exchange kernel

Andreas Görling and Andreas Hesselmann

Lehrstuhl für Theoretische Chemie, Universität Erlangen-Nürnberg, Erlangen, Germany

The random phase approximation (RPA) correlation energy is expressed in terms of the exact local Kohn-Sham (KS) exchange potential and corresponding adiabatic and nonadiabatic exchange kernels for density-functional reference determinants. The approach naturally extends the RPA method in which, conventionally, only Coulomb interactions are included. By comparison with the coupled cluster singles doubles with perturbative triples method it is shown for a set of small molecules that the new RPA method based on the KS exchange kernel yields correlation energies more accurate than RPA on the basis of Hartree-Fock exchange.

References

[1] A. Hesselmann and A. Görling, Mol. Phys., in press.

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Large-scale Second RPA calculations'

Panagiota Papakonstantinou

Institute of Nuclear Physics, T.U. Darmstadt, Schlossgartenstr. 9, 64289 Darmstadt, Germany

Extended RPA theories, which go beyond first-order RPA, are often used to describe the strength, decay width and fine structure of collective excitations in nuclei and other many-body systems. Second RPA (SRPA) is a simple and straightforward extension of RPA to second order, based on the Hartree-Fock ground state. In the present work, large-scale (i.e., without arbitrary truncations of the 1p1h+2p2h model space), “self-consistent” (i.e., with a single two-body interaction as the sole input) SRPA calculations are performed and analyzed. Divergence problems are avoided thanks to the use of finite-range interactions. It is presented how the large-scale eigenvalue problems that SRPA entails can be treated, and how the method operates in producing self-energy corrections and fragmentation. Besides the usual inconsistency problems (broken symmetries), stability problems are encountered, which are traced back to missing ground-state correlations. Nevertheless, nuclear giant resonances appear rather stable with respect to correlations. The results are discussed in the context of extended RPA theories, as well as of suitable effective interactions.

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