  The purpose of this paper is to derive the dynamical equations for the period
vectors of a periodic system under constant external stress. The explicit
starting point is Newton's second law applied to halves of the system. Later
statistics over indistinguishable translated states and forces associated with
transport of momentum are applied to the resulting dynamical equations. In the
final expressions, the period vectors are driven by the imbalance between
internal and external stresses. The internal stress is shown to have both full
interaction and kinetic-energy terms.
