High energy in NPT compared to NVT

Dear lammpsusers,

I am simulating films of Al2O3 which is periodic in x and y directions and nonperiodic in z direction. I have put vacuum above and below the crystal in z direction. I heat the system from 300 K to 1500 K. I have well equilibrated the crystal prior to heating .

When I use NVT, to increase the temperature, I get lower values of energy as compared to NPT for all temperature values. The energy values for NPT case is too high.

What could be the possible cause for high energy in NPT?

Are periodic boundary conditions in NPT causing atoms to come very close to one another resulting in high energy ?.

I would be grateful if someone can provide insight to it.

Thanks

NIkhil

Dear lammpsusers,

I am simulating films of Al2O3 which is periodic in x and y directions and
nonperiodic in z direction. I have put vacuum above and below the crystal in
z direction. I heat the system from 300 K to 1500 K. I have well
equilibrated the crystal prior to heating .

When I use NVT, to increase the temperature, I get lower values of energy as
compared to NPT for all temperature values. The energy values for NPT case
is too high.
What could be the possible cause for high energy in NPT?

there is not enough information here to even speculate about this.
you don't even say what kinds of energy you are comparing.

Are periodic boundary conditions in NPT causing atoms to come very close to
one another resulting in high energy ?.

what is that supposed to mean?

axel.

Did you check whether your crystal holds its structure during the simulation (using visualization software like VMD)?

If the crystal structure broke, the energy of the system would rise abnormally. For example, you may see that the atoms get closer together and form a new amorphous structure which has higher potential energy due to shorter distances between the atoms.

Regards
MN