@article{article59f71049,
title   = {Rayleigh-ritz variational principle for ensembles of fractionally occupied states},
author = {Gross, E K U and Oliveira, Luiz Nunes de and Kohn, W},
year = {1988},
doi = {10.1103/physreva.37.2805},
journal   = {Physical Review a}
}