I use Materials Studio software (V. 4.3). I have added polymeric matrix between two layers of MMT. For minimization and dynamics runs, I need to fix the MMT atoms (using Modify>> constraints>> atom>> fix fractional positions) and free the polymeric matrix to fluctuate. Also, I have to fix the a and b lattice parameters and free the c parameter to adjust during the NPT dynamics runs. Actually, I do all of the mentioned items, but the final structures shows that constraint has not been done successfully. Both of the atom positions and a, b , and c parameters of lattice change obviously. I would appreciate any one help to solve this problem.