Motivated by a problem arising from Density Functional Theory, we provide the sharp next-order asymptotics for a class of multimarginal optimal transport problems with cost given by singular, long-range pairwise interaction potentials. More precisely, we consider an N -marginal optimal transport problem with N equal marginals supported on R d and with cost of the form i =j |x i − x j | −s . In this setting we determine the second-order term in the N → ∞ asymptotic expansion of the minimum energy, for the long-range interactions corresponding to all exponents 0 < s < d. We also prove a small oscillations property for this second-order energy term. Our results can be extended to a larger class of models than power-law-type radial costs, such as non-rotationally-invariant costs. The key ingredient and main novelty in our proofs is a robust extension and simplification of the Fefferman-Gregg decomposition [F85], [Ge89], extended here to our class of kernels, and which provides a unified method valid across our full range of exponents. Our first result generalizes a recent work of Lewin, Lieb and Seiringer [LLS17], who dealt with the second-order term for the Coulomb case s = 1, d = 3 .1 and we will prove the following asymptotic expansion for F OT N,c (µ). For all 0 < s < d, if µ has density ρ ∈ L 1+s/d (R d ), then we have (see Theorem 1.1 below)Moreover we will prove that the strictly positive constant C(s, d) is independent of the choice of the marginal dµ(x) = ρ(x)dx and therefore can be interpreted as arising in an independent model problem. Furthermore, we will derive a small oscillations principle (with respect to N ) forwhich could also be interpreted as a rough third order asymptotic bound for F OT N,c (µ) with c as in (1.3). Our methods are extendable to more general costs, as non-rotationally-invariant ones. See Remark 1.3 below.For d = 3, s = 1 , the F OT N,c (µ) was introduced in the physics literature in the context of Density Functional Theory (DFT) by Seidl, Perdew, Levy, Gori-Giorgi, and Savin [Sd99, SPL99, SGGS07], without them being aware of its meaning in optimal transport. Namely, for s = 1, d = 3 , (1.1) is a natural semiclassical limit to the famous Hohenberg-Kohn (HK) functional from quantum mechanics, originally introduced by Hohenberg-Kohn in [HK64], and rigorously proved by Levy and Lieb in [Ly79], [Li83]. The connection to optimal transport was mathematically established later by [CFK11] and [BdPGG12] for N = 2 , later further extended to N = 3 in [BdP17], and recently independently proved for all N ≥ 2 by [CFK17] and [Lw17].In the process of establishing our two main results, we were required to prove, as key new tools for them, a set of additional secondary results of independent interest and of possible use to other settings, leading to generalized versions of the Fefferman-Gregg decomposition and positive definiteness criteria, as described in Section 4.1. What we use is a decomposition of positive definite kernels following the strategy established for s = 1, d = 3 by Fefferman [F85],...