When all the atoms are moved by the same amount, i.e. the crystal is
rigidly shifted, the force on each atom must be zero. This is a
stronger constraint than the one in which it is the sum of the
forces on each atom to be zero. The latter is expressed by:
The former condition is:
The constraint in Eq. 10 has to be imposed in such a way that the force constant matrix remains symmetric: . In the PHON code this is done iteratively, in a number of steps in which the symmetry is re-imposed at each step.
To impose translational invariance as described
set the variable:
NTI = 20
Translational invariance is imposed iteratively, and iterations are usually enough.
The amount of output printed by the PHON is controlled by the variable IPRINT. IPRINT=0 will produce a minimal output, IPRINT=3 a verbose output, which also includes the dynamical matrix and its eigenvectors.