## Exact first-order effect of interactions on the ground-state energy of harmonically-confined fermions

Pierre Le Doussal, Naftali R. Smith, Nathan Argaman

SciPost Phys. 17, 038 (2024) · published 7 August 2024

- doi: 10.21468/SciPostPhys.17.2.038
- Submissions/Reports

### Abstract

We consider a system of $N$ spinless fermions, interacting with each other via a power-law interaction $\epsilon/r^n$, and trapped in an external harmonic potential $V(r) = r^2/2$, in $d=1,2,3$ dimensions. For any $0 < n < d+2$, we obtain the ground-state energy $E_N$ of the system perturbatively in $\epsilon$, $E_{N}=E_{N}^{(0)}+\epsilon E_{N}^{(1)}+O(\epsilon^{2})$. We calculate $E_{N}^{(1)}$ exactly, assuming that $N$ is such that the "outer shell" is filled. For the case of $n=1$ (corresponding to a Coulomb interaction for $d=3$), we extract the $N \gg 1$ behavior of $E_{N}^{(1)}$, focusing on the corrections to the exchange term with respect to the leading-order term that is predicted from the local density approximation applied to the Thomas-Fermi approximate density distribution. The leading correction contains a logarithmic divergence, and is of particular importance in the context of density functional theory. We also study the effect of the interactions on the fermions' spatial density. Finally, we find that our result for $E_{N}^{(1)}$ significantly simplifies in the case where $n$ is even.

### Authors / Affiliations: mappings to Contributors and Organizations

See all Organizations.-
^{1}Pierre Le Doussal, -
^{2}Naftali R. Smith, -
^{2}^{3}Nathan Argaman

^{1}Laboratoire de Physique de l’École Normale Supérieure / Physics Laboratory of the École Normale Supérieure [LPENS]^{2}אוניברסיטת בן-גוריון בנגב / Ben-Gurion University of the Negev [BGU]^{3}קריה למחקר גרעיני – נגב ע"ש שמעון פרס / Shimon Peres Negev Nuclear Research Center [NRCN]