integer, dimension(6) :: cell_constraints

This array stores the constraints relating the cell lattice vectors which will be preserved during relaxations and MD.

The first three elements represent the magnitudes of the three lattice vectors ${\bf a}, {\bf b},{\bf c}$ and elements 4 through 6 represent constraints relating the angles between these vectors, $\alpha, \beta, \gamma$.

If the value of the element corresponding to a magnitude or angle is zero, this quantity will remain fixed. If two or three elements contain the same integer, the corresponding quantities will be constrained to have the same value. If a positive integer greater than 0 occurs in elements 1 through 3 the same integer cannot occur in elements 4 through 6 as this would imply that a vector length and angle must have the same value.