Descripency in energy of non-interacting molecules

Dear all,

I am trying to simulate water (Ice-Ih) molecules with only harmonic energy
as the interaction potential(atoms are attached to its lattice positions).
I am doing NVT simulation at 255K with no intermolecular interactions.
Since there is no interaction between molecules i expected to get same
average energy with single molecule and multiple molecules.

If single molecule energy is u1, for n molecules the energy should be n*u1

here i observed difference in the average energies for single molecule
0.743705 and for 10 molecules 1.44545 kcal/mol

can any one tell me the reason why is it so ?

RAVI KUMAR REDDY A
IISc Bangalore

Dear all,

I am trying to simulate water (Ice-Ih) molecules with only harmonic energy
as the interaction potential(atoms are attached to its lattice positions).
I am doing NVT simulation at 255K with no intermolecular interactions.
Since there is no interaction between molecules i expected to get same
average energy with single molecule and multiple molecules.

If single molecule energy is u1, for n molecules the energy should be n*u1

here i observed difference in the average energies for single molecule
0.743705 and for 10 molecules 1.44545 kcal/mol

can any one tell me the reason why is it so ?

‚Äčeither because of your input file or because of how you do the averaging
or both.

axel.‚Äč

Dear Axel,

please find my input scripts for both cases in the attachment.

averaging i have done with lammps only using fix ave/time

RAVI KUMAR REDDY A
IISc Bangalore

input-real.txt (1.03 KB)

conf-Ih.txt (686 Bytes)

input-real-10.txt (1.03 KB)

conf-Ih-10.txt (2.9 KB)