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: = <U*N> + <P/rho>. I have also tried = <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

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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