Molecules movement during NVT

Dear all,

During the run for NVT, the system of lipid bilayer in a big box which was not filled with any solvent had changes as the molecules distributed across the box. The system totally ruined and the I did not continue the other steps.

It seemed that the posres.itp were correctly inserted and a vaccum simulation is aimed.

Could you please tell me where the problem can be?

I appreciate your kindness in advance.