I would like to make sure that my understanding of internal energy is correct. I'm not a thermodynamicist, so apologies in advance for what is probably a basic question.

I previously calculated density for a liquid system via molecular dynamics (NPT ensemble). I needed the results for use in fluid flow simulations. The calculations also generated internal energy, which I have so far ignored. However, someone has expressed interest in comparing my internal energy results with his. He looked at the same system, but used a different calculation method.

His internal energies, E, for all temperatures and pressures are between -11000 to -12000 kJ/mol. Mine are between -38000 and -40000 kJ/mol.

Here's my question. Is it correct that I don't need to worry that one of us has made a mistake--that it's OK for the absolute values to differ as long as the differences are consistent? For example, his highest-temperature, highest-pressure data point differs from his lowest-temperature, lowest-pressure data point by about the same amount as mine does.

  • 1
    $\begingroup$ Generally, absolute energies aren't meaningful and we are mainly interested in energy differences. The concern I would have in comparing your results is that yours seem to have a much larger range (2000 vs 1000). $\endgroup$
    – Tyberius
    Aug 6 '20 at 17:13
  • $\begingroup$ @Tyberius, thanks very much for helping with such a basic question. If you want to put that in an answer, I will mark it correct. $\endgroup$
    – NTS
    Aug 6 '20 at 17:33
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    $\begingroup$ You may want to post this question at the material modeling SE site instead (I think Tyberius for one is a regular there). $\endgroup$
    – Buck Thorn
    Aug 6 '20 at 19:30
  • $\begingroup$ @BuckThorn, thanks. I'll do that if I haven't heard back from him by tomorrow--don't want him to think I deleted his current effort in case he checks back today! $\endgroup$
    – NTS
    Aug 6 '20 at 20:06

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