Dear LAMMPS Users,

I wanted to calculate the interfacial energy between two slabs.

Therefore, I made two groups (i.e. top and bottom)

variable zmiddle equal 101.868467157

region topregion block INF INF INF INF ${zmiddle} INF units box

region topregion block INF INF INF INF 101.868467157 INF units box

group top region topregion

4480 atoms in group top

group bottom subtract all top

4480 atoms in group bottom

After that, I tried two methods to calculate interfacial energy

Einterface = Etotal – Etop – Ebottom

(In my system vdw and long range coulombic interactions are the only energy contributions. I used Buck/coul/long)

First one was to use potential energy calculation.

compute topperatom top pe/atom pair kspace

compute toppe top reduce sum c_topperatom

compute bottomperatom bottom pe/atom pair kspace

compute bottompe bottom reduce sum c_bottomperatom

However, when I checked the values

pe = c_toppe + c_bottompe

Therefore, pe – c_toppe – c_bottompe = 0.

Second one was to use group/group command

compute einter bottom group/group top pair yes kspace yes boundary yes

c_inter had finite value, which is different from the first case.

Why do they show different results? Which one is what I want to calculate??

Thank you very much.

Best regards,

Sehun Joo.