I already answered your question. If you prefer words like "extensive
thermodynamic property," please consider your favorite equations from
thermodynamics, in which a linear function of energies, values of PV or
TS are present. On the other side, despite the fact that all of these
equations have been traditionally rescaled to 1 mol, there is total mass
of the system, i.e. what you call "extensive thermodynamic properties"
are nothing but properties of a given amount of matter.
The same logic explains the difference between total volume and volume
per mole. Your energy of intermolecular interactions is ~ 170 kJ/mol.
Alex
On 8/26/2018 10:12 PM, Dhawal Shah wrote:
Dear Alex,
Thank you for your response. I agree with all your statements.
My issues are with units. In the first case, should it be kJ (total
energy), as it is the Total potential energy_of the system_? Total energy
of the system is an extensive thermodynamic property.
I mean, for example, volume. We have total volume of the system and volume
per mole. Both are different. If I say that 100 moles of water have 1000 m3
(Total volume_of the system_), and per mole, it is 10m3/mol. One is
system-size dependent and second one is not.
Regards,
Dhawal
The potential energy _of the system_ is reported, and that's directly
proportional to the number of molecules you have in your box. Energy per
mole is a unit regardless of the number of molecules. After you properly
rescale the total energy with the number of molecules, you still get
kJ/mol, of course. In one case, it was per the entire system containing
N molecules, in the other it is per molecule. You do not weigh x kilos
per Dhawal, you just weigh x kilos.
Alex
On 8/26/2018 9:02 PM, Dhawal Shah wrote:
Greetings!
I am confused about the units of potential energy in gromacs. I have
performed simulations of pure monoethanol (MEA) systems with certain
parameters. The three systems I simulated consisted of 2400, 2800, and
3200
molecules of MEA, respectively, at 298 K and 1 bar. After reaching
equilibrium, the potential energy of the systems (as computed from gmx
energy were)
Number of MEA molecules
MEA-MEA interaction energy (kJ/mol)
In pure MEA
2400
409523.0
2800
477119.5
3200
545348.3
How, can the potential energy (with units kJ/mol) be different for
different systems. It is all pure MEA, and the systems are in equilibrium.
Also the units is per mole. Energy per mole should be independent of the
number of molecules simulated?
However, if I divide the energy by the number of molecules, it is roughly
the same. 409523/2400 = 170.63; 477119.5/2800 = 170.39; 545348.3/3200 =
170.42. What is the unit now = kJ/mole/molecules?
If I want to understand the strength of interactions between MEA
molecules,
should I choose 170 kJ/mol/molecules or 409523 kJ/mol?
Regards,
Dhawal
*Dhawal Shah*
*Assistant Professor*
*Chemical Engineering Department*
*Nazarbayev University, Astana*
*Phone: +7 7172 70 9130 (O)*
*email: dhawal.s...@nu.edu.kz <dhawal.s...@nu.edu.kz>*
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