[lammps-users] Enthalpy Change Calculations

Hi LAMMPS users,

I am posting this question again to see if anyone has any new help with my problem:

Has anyone successfully used LAMMPS to calculate the sensible enthalpy of a substance? I am attempting to calculate sensible enthalpy for a propellant I am researching, but I am getting calculated results that are up to 50% off. My simulations predict the condensed-phase density of the substance accurately, but are significantly off on enthalpy.

I have also tried a simple argon example, but am still significantly off on my enthalpy change prediction.

I am calculating the enthalpy of a substance (using an NPT run) by getting the average E_total (from the LAMMPS output) and adding the average pressure divided by the average density: = <UN> + <P/rho>. I have also tried = <UN + P/rho>.

Where H is the computed enthalpy, U is the total energy/atom (using ‘thermo_modify norm yes’), N is the number of atoms per molecule – for argon N=1, P is the pressure calculated by LAMMPS and rho is the density calculated by LAMMPS.

I have included the long-range pressure correction for the LJ term and the long-range pressure and energy corrections for the LJ and coulombic terms. I have tested out these long-range corrections to make sure they work correctly and they seem to be working fine.

I perform two different P-T combinations and calculate the difference in enthalpy between the two runs. This difference is typically 30-50% larger than experimental values even when the predicted densities for the two runs are very close to the experimental values. When the substances are in the gas phase, the enthalpy difference is very accurate (when the KE dominates the total energy term and there is very little PE). However, when the PE term becomes more significant (in the condensed phases), my enthalpy change predictions are incorrect.

I have tried using both ‘real’ and ‘lj’ units with no success.

Any help would be appreciated.

Thanks,

Tim Kokan

Hi Tim. I think that the disagreement between your
calculations and the experimental measurements are due
to force field inadequacies. This type of calculation
would indeed be sensitive to the shape of the
potential surface. I've asked Marcus Martin about this
(he's the main author of the Towhee MC code:
http://towhee.sourceforge.net/ ), and he suggests that
you perform a phase equilibrium calculation with his
code to see if the force field you're using predicts
the correct enthalpy of vaporization. Marcus also
suggested that you use the setpoint pressure in your
enthalpy calculation rather than the output pressure
since the output pressure for liquids is noisy. You
could also do an NPT simulation using his code and
compare to the densities you're getting with LAMMPS to
ensure that the tail correction code is working right.
Marcus may have more to add to this disucssion, so
I'll CC him on this message. I hope these suggestions
help! -Paul

Hi LAMMPS users,

I am posting this question again to see if anyone
has any new help with my
problem:

Has anyone successfully used LAMMPS to calculate the
sensible enthalpy of a
substance? I am attempting to calculate sensible
enthalpy for a propellant
I am researching, but I am getting calculated
results that are up to 50%
off. My simulations predict the condensed-phase
density of the substance
accurately, but are significantly off on enthalpy.

I have also tried a simple argon example, but am
still significantly off on
my enthalpy change prediction.

I am calculating the enthalpy of a substance (using
an NPT run) by getting
the average E_total (from the LAMMPS output) and
adding the average pressure
divided by the average density: <H> = <U*N> +
<P/rho>. I have also tried
<H> = <U*N + P/rho>.

Where H is the computed enthalpy, U is the total
energy/atom (using
'thermo_modify norm yes'), N is the number of atoms
per molecule - for argon
N=1, P is the pressure calculated by LAMMPS and rho
is the density
calculated by LAMMPS.

I have included the long-range pressure correction
for the LJ term and the
long-range pressure and energy corrections for the
LJ and coulombic terms.
I have tested out these long-range corrections to
make sure they work
correctly and they seem to be working fine.

I perform two different P-T combinations and
calculate the difference in
enthalpy between the two runs. This difference is
typically 30-50% larger
than experimental values even when the predicted
densities for the two runs
are very close to the experimental values. When the
substances are in the
gas phase, the enthalpy difference is very accurate
(when the KE dominates
the total energy term and there is very little PE).
However, when the PE
term becomes more significant (in the condensed
phases), my enthalpy change
predictions are incorrect.

I have tried using both 'real' and 'lj' units with
no success.

Any help would be appreciated.

Thanks,

Tim Kokan

_________________________________

Tim Kokan

Space Systems Design Lab (SSDL)
Georgia Institute of Technology

E-mail: [email protected]
Phone: 404-894-7783 (lab)

404-849-7368 (cell)

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