How to add coupled boundary condition ?

Hi

I am working on a diffusion/stress coupled problem by using species transport/solid mechanics modulues in COMSOL.

I tried to add a concentration BC, which is a function of the hydrostatic stress (hds):

c=f(hds, P)------------------------------(1)

hds is defined in the variable, which equals to the average of three principal stresses and is dependent on the position. P is the gas pressure, which is dependent on the species flux. Everything is fine when the stress effect is not included:

c=f(P)------------------------------------(2)

But the software shutdown immediately when the stress effect is added, i.e. equation (1).

Does anyone know the solution ？

Much appreciated !

Yafei