Lattice dynamics calculations have been performed for beta-9,10-dichlo
roanthracene and 9,10-dibromoanthracene under pressure. The potential
has been fitted to the ambient pressure crystallographic structure and
to lattice phonon frequencies of both compounds. The pressure depende
nce of the structures and frequencies has been calculated. A pressure
induced phase transition which resembles the experimental transition i
s predicted for the chloro derivative at approximate to 3 GPa.