Calculation of enthalp

Dear Lammps/MD Expert,

I am going to calculate the enthalp of deformation(H=U+PV) inside my 2 dimensional model with LAMMPS. for this purpose:

1- first I calculate the internal energy of the region (not all the system):
-variable etotal_region equal “(c_pe_region+c_ke_region)”
2- Then I calculate the pressure inside the 2 dimensional region with this command:

• variable press_region equal -(c_p[1]+c_p[2])/(2*Area of the region)
3- then I calculate the enthalpy in my NVE ensemble with :
-variable enthalpy equal "(v_etotal+ Area of the region *v_press_region)

and I reached to some results.

Howeve, I have no idea what should be the order of the enthalpy values actually!!! I guess they should be with the same order with internal energy.

Best regards,

Hi Bahman,

What you are doing is conceptually straight forward, but it is not a typical LAMMPS calculation, and so there are lots of extra things you have to get right. I suggest eliminating some of the complicating factors first, reproducing a known correct result, and then adding back in the complicating factors one by one.

1. A related LAMMPS calculation would be to calculate total internal energy as function of compression for a 3D structure. This could be done statically at zero temperature, so no kinetic contribution and no configurational sampling, or it could be done at finite temperature, with a kinetic contribution, and time-averaging of the configurational distribution. So, first figure out how to do that calculation correctly, comparing to prior results.

2. Restrict the calculation to a finite region. This is a non-trivial extension. For example, the c_p[1] that you use below must be defined for a region, but the LAMMPS compute pressure always operates on the group all (From the doc page 'The specified group must be “all”. '). You will have to use stress/atom or else make some kind of assumption about the relation between pressure in region and global pressure.

3. Similar considerations apply to defining kinetic energy on a region.

4. Restrict to 2D

Since you say that you have no idea what values to expect, I strongly suggest you spend some time reading the prior literature, to get some idea.

Aidan