PACS 05.10.Ln -Monte Carlo methods PACS 75.10.Nr -Spin-glass and other random models PACS 05.20.-y -Classical statistical mechanicsAbstract -We derive analytically the full distribution of the ground-state energy of K noninteracting fermions in a disordered environment, modelled by a Hamiltonian whose spectrum consists of N i.i.d. random energy levels with distribution p(ε) (with ε ≥ 0), in the same spirit as the "Random Energy Model". We show that for each fixed K, the distribution PK,N (E0) of the ground-state energy E0 has a universal scaling form in the limit of large N . We compute this universal scaling function and show that it depends only on K and the exponent α characterizing the small ε behaviour of p(ε) ∼ ε α . We compared the analytical predictions with results from numerical simulations. For this purpose we employed a sophisticated importance-sampling algorithm that allowed us to obtain the distributions over a large range of the support down to probabilities as small as 10 −160 . We found asymptotically a very good agreement between analytical predictions and numerical results.The celebrated "Random Energy Model" (REM) of Derrida [1] has continued to play a central role in understanding different aspects of classical disordered systems, including spin-glasses, directed polymers in random media and many other systems. In the REM, one typically has N energy levels which are considered to be independent and identically distributed (i.i.d.) random variables, each drawn from a probability distribution function (PDF) p(ε). Typical observables of interest are the partition function, free energy, etc. The REM can also be useful as a toy model in quantum disordered systems. For example, let us consider a single quantum particle in a disordered medium with the Hamiltonianĥ. We will assume that the spectrum of the operatorĥ has a finite number of states N (for instance a quantum particle on a lattice of finite size and a random onsite potential, as in the Anderson model). In general, solving exactly the spectrum of such an operator is hard, for a generic random potential. One possible approximation, in the spirit of the REM in classical disordered systems, would be to consider the toy model where one replaces the spectrum of the actual Hamiltonian by N ordered i.i.d. energy levels ε 1 ≤ ε 2 ≤ · · · ≤ ε N each drawn from the common PDF p(ε). Without loss of generality, we will also assume that the Hamiltonianĥ has only positive eigenvalues. This would mean that, in the corresponding toy model, the PDF p(ε) is supported on [0, +∞). It is well known that, in a strongly disordered quantum system, where all single-particle eigenfunctions are localised in space, the energy levels can be approximated by i.i.d. random variables (see e.g. [2]). Therefore the p-1 arXiv:1808.09246v1 [cond-mat.dis-nn]