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

We consider a system of N N spinless fermions, interacting with each other via a power-law interaction ε/r^n ϵ/rn , and trapped in an external harmonic potential V(r) = r^2/2 V(r)=r2/2 , in d=1,2,3 d=1,2,3 dimensions. For any 0 < n < d+2 0<n<d+2 , we obtain the ground-state energy E_N EN of the system perturbatively in ε ϵ , EN=EN≤ft(0)+ε EN≤ft(1)+O≤ft(ε²) EN=EN≤ft(0)+ϵEN≤ft(1)+O≤ft(ϵ2) . We calculate EN≤ft(1) EN≤ft(1) exactly, assuming that N N is such that the “outer shell” is filled. For the case of n=1 n=1 (corresponding to a Coulomb interaction for d=3 d=3 ), we extract the N 1 N≫1 behavior of EN≤ft(1) EN≤ft(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 EN≤ft(1) EN≤ft(1) significantly simplifies in the case where n n is even.