Rigid molecule approximation for molecular crystals

Hi, I started to use GULP for the phonon structure of organic molecular crystals, e.g., pentacene. One convenient approximation to start with is to use the rigid molecule approximation, i.e., the intramolecular bonds are fixed in their equilibrium position, and only the intermolecular phonons are calculated. There is convenient way of doing that with GULP ? I partly achieved this by separating intra and inter-molecular atom-atom potentials, and giving the force constants of the former very high values. But perhaps there is better way…the rigid options does not seem to be applicable to crystals.

Apologies for the delay in the reply but I’ve been on leave. In versions 5.2 and earlier rigid molecules are only possible for Monte Carlo due to the pack of forces for rigid bodies. I will shortly release version 6.0 and this will support rigid molecules for optimisation and property calculations (not yet MD) & so this should resolve your issue. I hope to make the new release available in January.

Thank you Julian. I will look forward for the new release. Best wishes