Coefficient Form PDE and periodic boundary conditions

Hi,

I'm new to COMSOL so I would like some help regarding the following -

I'm trying to perform a phononic crystal simulation using coefficient form pde. I have a hexagon with a circular hole shape removed - this is my geometry.

I want to apply a zero flux boundary condition at the circular shape and periodic boundary conditions at opposing edges of the hexagon.

So I end up using 3 periodic boundary conditions, for the 3 pairs of opposing sides of the hexagon. Unfortunately COMSOL sets up zero flux boundary conditions by default on all the boundaries and I don't see how I can remove this boundary condition on the hexagon's edges on which I only want the periodic boundary conditions.

Any help would be greatly appreciated!

Kartiek.
I'm using COMSOL 4.0a