Analytical Hartree-Fock gradients with respect to the cell parameter for   systems periodic in three dimensions
  Analytical Hartree-Fock gradients with respect to the cell parameter have been implemented in the electronic structure code CRYSTAL, for the case of three-dimensional periodicity. The code is based on Gaussian type orbitals, and the summation of the Coulomb energy is performed with the Ewald method. It is shown that a high accuracy of the cell gradient can be achieved. 
K. Doll, R. Dovesi, R. Orlando