  We formulate a doubly nonnegative (DNN) relaxation of the protein side-chain
positioning (SCP) problem. We inherit the natural splitting of variables that
stems from the facial reduction technique in the semidefinite relaxation. We
solve the relaxation using a variant of the Peaceman-Rachford splitting method.
Our numerical experiments show that we solve almost all instances of the
NP-hard SCP problem to optimality.
