  The variational calculation of the two-electron reduced density matrix
(2-RDM) is extended to periodic molecular systems. If the 2-RDM theory is
extended to the periodic case without consideration of time-reversal symmetry,
however, it can yields energies that are significantly lower than the correct
energies. We derive and implement linear constraints that enforce time-reversal
symmetry on the 2-RDM without destroying its computationally favorable
block-diagonal structure from translational invariance. Time-reversal symmetry
is distinct from space-group or spin (SU(2)) symmetries which can be expressed
by unitary transformations. The time-reversal symmetry constraints are
demonstrated through calculations of the metallic hydrogen chain and the
one-dimensional lithium hydride crystal.
