# How to reduce enthalpy in an NPH ensemble?

Bo,
Please, always "reply to all" to keep the list aware of the
discussion. This will improve your chances of hopefully being helped
by more knowledgeable people.
Take my next comments with skepticism as I am far fro being an expert
in this topic. The last phase transition research I did was years ago
and never used NPH ensembles.

Charlos,
Thanks for your reply! Just as you said, I tried the "fix temp/rescale"
command to rescale the temperature of the system of liquid Si in a series of
consecutive NPH runs, each NPH run consists of a former temperature
rescaling (10000 steps, 15ps) and a latter singlely NPH equibration (2000000
step, 3ns). In each next run, the "Tstart" was set the value averaged from
the equibration part of its former run, and "Tstop" was set as "Tstart-5",
but I encountered some difficulties:

I am a bit confused by your reference to Tstart and Tstop. Do you have
a thermostat coupled to
the system? I don't think that's what you are suppose to do (if that
is indeed what you did). The idea
is just to rescale the velocities at time=0 and let the system
equilibrate. Then compute the temperature (T) after equilibration. As
long as you have kinetic energy in your system you have temperature.
For example
an NVE ensemble has a well define T in the absence of a thermostat.
Next, repeat this process in a series of steps where the input of each
run is the equilibrated system of the previous one. Each run gives you
a (T,H) pair that you then plot to search for the transition. The
monotonic behavior of that curve should aid you identify the
transition. There is no T changing in time during each simulation (in
the production part). That's my take from my quick read to the
reference I sent.
Maybe someone else with more tangible expertise in this particular
Carlos

PS: I like the "Charlos" instead of Carlos in your previous response.
Was that a mix of Charles and Carlos?

Carlos,
I’m really sorry for my caleless to mistake your name, and I thank you very much for your detailed instruction, I think I’ve learned a lot from you. You suggestion of “just to rescale the velocities” is right what I was intended to do, but I couldn’t find a command in LAMMPS to do it! The most likely one I found was the “fix temp/rescale” command, which as you said, is not the right way to rescale the velocities. Could you help me with a proper command or method to rescale the velocities in LAMMPS?
Bests,
Bo Shen

Carlos,
I'm really sorry for my caleless to mistake your name, and I thank
you very much for your detailed instruction, I think I've learned a lot
from you. You suggestion of "just to rescale the velocities" is right what
I was intended to do, but I couldn't find a command in LAMMPS to do it! The
most likely one I found was the "fix temp/rescale" command, which as you
said, is not the right way to rescale the velocities. Could you help me
with a proper command or method to rescale the velocities in LAMMPS?

http://lmgtfy.com/?q=velocity+rescale+lammps

...and feel very embarrassed.

axel.

Bo,
I never mentioned any specific Lammps command to be (or not to be)
used. I only said that the velocity rescaling takes place once, at the
beginning of the simulation.