New fluctuation properties arise in problems where both spatial integration and energy summation are necessary ingredients. The quintessential example is given by the short-range approximation to the first order ground state contribution of the residual Coulomb interaction. The dominant features come from the region near the boundary where there is an interplay between Friedel oscillations and fluctuations in the eigenstates. Quite naturally, the fluctuation scale is significantly enhanced for Neumann boundary conditions as compared to Dirichlet. Elements missing from random plane wave modeling of chaotic eigenstates lead surprisingly to significant errors, which can be corrected within a purely semiclassical approach. The characterization of quantum systems with any of a variety of underlying classical dynamics, ranging from diffusive to chaotic to regular, has often demonstrated that the study of their statistical properties is of primary importance. Spectral fluctuations are a principle example as they gave the first support to one of the main results linking classical chaos and random matrix theory [1], the Bohigas-Giannoni-Schmit conjecture [2,3]. Needless to say, the statistical properties of eigenfunctions are also a subject of paramount interest [4,5,6,7,8,9,10,11,12].For chaotic systems, a widely accepted starting point for the treatment of eigenfunction fluctuations locally, such as the amplitude distribution or the two-point correlation function c(|r − r ′ |) = ψ(r)ψ(r ′ ) of a given eigenfunction ψ, is a modeling in terms of a random superposition of plane waves (RPW) [4,5]. For two-degree-of-freedom systems, c(r) can be understood as being given approximately by a Bessel function. For distances |r − r ′ | short compared to the system size, and in the absence of effects related to classical dynamics [8,9,13], this is roughly observed in numerical [7,14,15] and experimental [16] studies. Our interest here is in statistical properties of eigenfunctions going beyond local quantities such as c(r).One motivation for the introduction of these new statistical measures is to study the interplay between interferences and interactions in mesoscopic systems. For typical electronic densities, the screening length is close to the Fermi wavelength λ F , and the screened Coulomb interaction can be approximated by the short range expression V sc (r − r ′ ) = (2ν) −1 F a 0 δ(r − r ′ ) with 2ν the mean local density of states, including spin degeneracy, (ν = m/2π 2 for d = 2) and To this level of approximation, the first order ground state energy contribution of the residual interactions can be expressed in the formwith N σ the unperturbed ground state density of particles with spin σ. From this expression, it is seen that the increase of interaction energy associated with the addition of an extra electron is related toand the understanding that E i < E + i < E i+1 . Our goal in this letter is to study the fluctuation properties of the S i , concentrating on the case of two dimensional billiards with either Dirichlet ...