Abstract
The present document focuses on the theoretical foundations of the nuclear energy density functional (EDF) method. As such, it does not aim at reviewing the status of the field, at covering all possible ramifications of the approach or at presenting recent achievements and applications. The objective is to provide a modern account of the nuclear EDF formalism that is at variance with traditional presentations that rely, at one point or another, on a Hamiltonian-based picture. The latter is not general enough to encompass what the nuclear EDF method represents as of today. Specifically, the traditional Hamiltonian-based picture does not allow one to grasp the difficulties associated with the fact that currently available parametrizations of the energy kernel E[g′,g] at play in the method do not derive from a genuine Hamilton operator, would the latter be effective. The method is formulated from the outset through the most general multi-reference, i.e. beyond mean-field, implementation such that the single-reference, i.e. “mean-field”, derives as a particular case. As such, a key point of the presentation provided here is to demonstrate that the multi-reference EDF method can indeed be formulated in a mathematically meaningful fashion even if E[g′,g] does not derive from a genuine Hamilton operator. In particular, the restoration of symmetries can be entirely formulated without making any reference to a projected state, i.e. within a genuine EDF framework. However, and as is illustrated in the present document, a mathematically meaningful formulation does not guarantee that the formalism is sound from a physical standpoint. The price at which the latter can be enforced as well in the future is eventually alluded to.
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Notes
- 1.
The notion of “controlled” description refers to the capability of estimating uncertainties of various origins in the theoretical method employed.
- 2.
In the sd shell for example, it is necessary to (slightly) refit about 30 combinations of two-body matrix elements in order to reach about 140 keV root mean square error on nearly 600 pieces of spectroscopic data [42].
- 3.
The adjective “diagrammatic” refers to many-body methods relying on the use of Feynman or Goldstone diagrams.
- 4.
I.e. the density-dependence of the effective Hamilton operator in the traditional formulation.
- 5.
Approximations such as the quasi-particle random phase approximation or the Schroedinger equation based on a collective (e.g. Bohr) Hamiltonian are only mentioned in passing; see Sect. 7.5.7.
- 6.
For certain symmetries, e.g. SO(3), the phase α collects in fact several angles. See Table 7.1 for two relevant examples.
- 7.
Although it can be done rigorously, we do not state explicitly here the definition of the norm of κ.
- 8.
Coulomb and center-of-mass correction contributions are omitted here for simplicity.
- 9.
Proton/neutron mixing is presently ignored such that \(\rho^{g' g} (\vec{r} \sigma\tau ,\vec{r} ' \sigma' \tau')=\kappa^{g' g} (\vec{r} \sigma\tau ,\vec{r} ' \sigma' \tau')=0\) for τ≠τ′. This does not correspond to the most general situation [86].
- 10.
In the case of the present toy functional, the fulfilment of Eq. (7.16) under Galilean transformations does not correlate any of the couplings.
- 11.
The present analysis can be easily extended to trilinear functional terms and effective three-body matrix elements.
- 12.
This encompasses the intermediate case where the EDF kernel is computed as the matrix elements of a density-dependent effective “Hamiltonian”. Indeed, in such a case no exchange or pairing term corresponding to the density dependence of the effective vertex appears in the EDF kernel.
- 13.
One way to ensure that the minimization is indeed performed within the manyfold of product states consists of adding an additional Lagrange constraint requiring that the generalized density matrix [51] ℛ remains idempotent.
- 14.
Expressions are given here for linear constraints although practical calculations often rely on quadratic constraints [107].
- 15.
Depending on the isospin projection τ considered, λ=λ n or λ p .
- 16.
It is specific to the EDF method to implicitly account for correlations via the functional character of E[g,g]. As such, one-nucleon separation energies \(E_{k}^{|g|\pm}\) obtained through SR-EDF calculations can be seen as effective centroids of a more fragmented underlying spectrum generated via a theory that explicitly accounts for those correlations.
- 17.
Inaccuracies associated with the quality of empirical EDF parametrizations are responsible for quantitative discrepancies while the present discussion relates to qualitative differences that are built in on purpose.
- 18.
- 19.
Such a procedure can be extended to any subgroup of 𝒢.
- 20.
Equation (7.52) can be recovered by expressing matrices \(\mathbf{S}_{k}^{(g) \pm}\) in a spherical basis p=nπjmτ and by working out how such matrices transform under the rotation of |Φ (g)〉 and \(| \varPhi^{(g)}_{k} \rangle\).
- 21.
If \(\mathbf{h}^{(\rho _{\lambda\mu} 0)}\) breaks parity, one further needs to extract the component belonging to the trivial Irreps of C i , i.e. the inversion center group. Indeed, restoring spherical symmetry does not ensure that parity is a good quantum number, e.g. a j=3/2 single-particle state can be a linear combination of d 3/2 and p 3/2 states. Proceeding to such an extraction would deliver a one-body field that is block-diagonal with respect to parity π as well.
- 22.
- 23.
It is important to underline at this point that the notion of “deformation” differs depending on the angular momentum of the targeted many-body state. This is due to the fact that a symmetry-conserving state with angular momentum J does display non-zero multipole moments of the density for λ≤2J [75]. For example, having a reference state with non-zero quadrupole and hexadecapole moments does not characterize a breaking of rotational symmetry if one means to describe a J=2 state. In such a case, one must check multipoles with λ>4 (or any odd multipole) to state whether rotational symmetry is broken or not. It happens that product states of the Bogoliubov type usually generate non-zero multipole moments of all (e.g. even) multipolarities as soon as they display a non-zero collective quadrupole moment. As such, they break rotational symmetry independent of the angular momentum of the good-symmetry state one is eventually after.
- 24.
Of course, the fact that the neutron or proton number is magic is not known a priori but is based on a posteriori observations and experimental facts. In particular, the fact that traditional magic numbers, i.e. N,Z=2,8,20,28,50,82,126, remain as one goes to very isospin-asymmetric nuclei is the subject of intense on-going experimental and theoretical investigations [10].
- 25.
The scheme can be extended to a set of local densities or to the full density matrix.
- 26.
We take advantage of property (7.16) to fix one of the two phases involved to zero.
- 27.
Such a mixing does not appear in the case of the U(1) group given that its Irreps are of dimension 1.
- 28.
The overlap kernel being analytical over the complex plane, it is straightforward to prove that 𝒩N=0 for N≤0.
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Acknowledgements
It is a great pleasure to thank deeply all those I have had the chance to collaborate with on topics related to the matter of the present lecture notes, i.e. B. Avez, M. Bender, K. Bennaceur, P. Bonche, B. A. Brown, P.-H. Heenen, D. Lacroix, T. Lesinski, J. Meyer, V. Rotival, J. Sadoudi, N. Schunck and C. Simenel. I also wish to thank M. Bender for providing me with several of the figures that are used in the present lecture notes.
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Appendix: F-Functions
Appendix: F-Functions
Kinetic densities are expressed in INM in terms of functions \(F^{(0)}_{m}(I_{\tau},I_{\sigma},I_{\sigma\tau})\), \(F^{(\tau)}_{m}(I_{\tau},I_{\sigma},I_{\sigma\tau})\), \(F^{(\sigma)}_{m}(I_{\tau},I_{\sigma},I_{\sigma\tau}) \) and \(F^{(\sigma\tau)}_{m}(I_{\tau},I_{\sigma},I_{\sigma\tau}) \) defined through [114]
Their first derivatives with respect to spin, isospin and spin-isospin excesses are
while their second derivatives are
for any i,j∈{0,τ,σ,στ}. Remarkable values are
and
where i∈{τ,σ,στ}.
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Duguet, T. (2014). The Nuclear Energy Density Functional Formalism. In: Scheidenberger, C., Pfützner, M. (eds) The Euroschool on Exotic Beams, Vol. IV. Lecture Notes in Physics, vol 879. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-45141-6_7
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