# melting point/problem with total energy

I want to calculate the melting point for tip4p.with plot of energy versus time

1. I have used this paper (The melting point of ice Ih for common water models calculated from direct coexistence of the solid-liquid interface) for comparing the results.there is a big difference between total energies. I have used total energy of system that calculated with thermo_style command.

The order of energies of mine is 1000 Kcal/mol, but theirs is 10 Kj/mol.(I have attached my results and my input file) . when I use the thermo_modify norm yes,The order of energy is more logical, but it is not correct yet(file is attached). My method for determining the melting point is it true?

1. I have used npt ensemble and p=1 (unit real) and constant temperature. It is been seen that the pressure fluctuates widely.(pressure file is attached ). I have used pchain and tchain and drag commands for solving the fluctuation, but it did not work.

Could you help me with these problem,please?

etotal.txt (211 Bytes)

input.in (2.43 KB)

I want to calculate the melting point for tip4p.with plot of energy
versus time

​why?​ there are much better methods to do this.

1) I have used this paper (The melting point of ice Ih for common water
models calculated from direct coexistence of the solid-liquid interface)

​this paper is using one of them.​

for comparing the results.there is a big difference between total
energies. I have used total energy of system that calculated with
thermo_style command.

​in all likeliness they will be different. please explain why you think
they should be the same and what you did when setting up your system to ​

The order of energies of mine is 1000 Kcal/mol, but theirs is 10
Kj/mol.(I have attached my results and my input file) . when I use the
thermo_modify norm yes,The order of energy is more logical,

​this has nothing to do with logic but rather with normalization. it looks
like you have some serious conceptual difficulties with this and i strongly
particularly let somebody explain to you the difference between intensive
and extensive properties.​

but it is not correct yet(file is attached). My method for determining
the melting point is it true?

2) I have used npt ensemble and p=1 (unit real) and constant temperature.
It is been seen that the pressure fluctuates widely.(pressure file is
attached ). I have used pchain and tchain and drag commands for solving the
fluctuation, but it did not work.
Could you help me with these problem,please?

​this topic has been discussed on this very mailing list only a gazillion
times. please dig through the archives. ​

Dear Axel
I looked at mailing list, but I can not find an appropriate solution.

Dear Axel
I looked at mailing list, but I can not find an appropriate solution.

​you didn't look well enough. ....and you have a paper *using* a better
solution right in front of you.​

yes. they use from this method:
The velocity at which the interface boundary moves as a
function of deviations from the equilibrium melting tempera-
ture is usually termed the interface response function.
I do not know how to do it with lammps. I looked for it . but I did not answer in lammps users.

I’m not sure what you are asking. Possibly
that you want to track the interface between a liquid
and solid automatically? For a crystalline system

(e.g. a metal lattice) you can often do this by
monitoring the local order around each atom. E.g.

computes like compute centro/atom or compute cna/atom
or even compute coord/atom. But for a molecular

crystal like ice, I think you will need to post-process

dump files and figure out the location of the interface

from each snapshot.

Steve