We develop a perturbation theory to efficiently calculate derivatives of the free energy with respect to changes in the parameters of the unit cell in the self-consistent-field theory (SCFT) of block copolymer crystals, thus determining the state of mechanical stress in any nonequilibrium unit cell. By requiring that the stress thus calculated vanish, we are able to find the unit cell that minimizes the free energy simultaneously with the solution of the SCFT equations, at little added computational effort.