Descripency in energy of non-interacting molecules

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1

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

2

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 ?

​either because of your input file or because of how you do the averaging
or both.

axel.​

3

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)