# Calculation of interaction energy between small molecules **URL:** \ **Category:** User discussions\ **Created:** [May 19, 2023, 7:18am UTC](https://gromacs.bioexcel.eu/t/calculation-of-interaction-energy-between-small-molecules/6445 "2023-05-19T07:18:07Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Dhrubajyoti](https://avatars.discourse-cdn.com/v4/letter/d/a5b964/32.png) [@Dhrubajyoti](https://gromacs.bioexcel.eu/u/Dhrubajyoti)\ **Post date:** [May 19, 2023, 7:18am UTC](https://gromacs.bioexcel.eu/t/calculation-of-interaction-energy-between-small-molecules/6445/1 "2023-05-19T07:18:07Z")
GROMACS version: 2018.3 GROMACS modification: No My system contains 512 ion pairs of Choline Chloride, 1024 EG, and 170 paracetamol molecules. I have calculated SR LJ and Coulomb energy between different energy groups. The SR LJ+Coul energy for different pairs are as follows: 1. -12000 kJ/mol for paracetamol-paracetamol 2. -17800 kJ/mol for choline chloride-paracetamol 3. -88800 kJ/mol for choline chloride-EG 4. -10300 kJ/mol for EG-paracetamol Are these values physically possible? I have often found much less energy values in literature. Am I missing something? Thanks in Advance Dhrubajyoti Maji ---
**Author:** ![hess](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/hess/32/416_2.png) [@hess](https://gromacs.bioexcel.eu/u/hess)\ **Post date:** [May 19, 2023, 8:39am UTC](https://gromacs.bioexcel.eu/t/calculation-of-interaction-energy-between-small-molecules/6445/2 "2023-05-19T08:39:01Z")
This could be reasonable numbers. But interaction energies are nearly meaningless for most purposes. I don’t know what literature values you are referring to, but most relative interaction free-energies are usually what matters. So there contain energy differences, not absolute interaction energies and also contain entropic contributions. ---
**Author:** ![Dhrubajyoti](https://avatars.discourse-cdn.com/v4/letter/d/a5b964/32.png) [@Dhrubajyoti](https://gromacs.bioexcel.eu/u/Dhrubajyoti)\ **Post date:** [May 19, 2023, 9:02am UTC](https://gromacs.bioexcel.eu/t/calculation-of-interaction-energy-between-small-molecules/6445/3 "2023-05-19T09:02:44Z")
Many thanks for your reply. Some papers report this summation of sort range Coulomb and LJ energies as ‘interaction energy’. They studied the variation of this IE across temperature or composition. Here I’m trying to calculate the same by changing the number of paracetamol molecules. But the reported values are in the range of 300 kJ/mol whereas for my system values are too high. This is where confusion arises. ---
**Author:** ![hess](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/hess/32/416_2.png) [@hess](https://gromacs.bioexcel.eu/u/hess)\ **Post date:** [May 19, 2023, 9:33am UTC](https://gromacs.bioexcel.eu/t/calculation-of-interaction-energy-between-small-molecules/6445/4 "2023-05-19T09:33:51Z")
I don’t believe such approaches are useful. But isn’t the issue simply that you get the sum over all molecules, where the literature value is per molecule? (whatever that might mean, the results would still depend on concentration) ---
**Author:** ![Bauyrzhan](https://avatars.discourse-cdn.com/v4/letter/b/4bbf92/32.png) [@Bauyrzhan](https://gromacs.bioexcel.eu/u/Bauyrzhan)\ **Post date:** [May 25, 2023, 3:54am UTC](https://gromacs.bioexcel.eu/t/calculation-of-interaction-energy-between-small-molecules/6445/5 "2023-05-25T03:54:29Z")
Dear Mr. Dhrubajyoti, I was interested in your MD simulation. I am also trying MD simulation of the Choline chloride and Ethylene glycol system. But, I am having trouble with that. Could you give some advices on that, please? My email is: [bauyrzhan.myrzakhmetov86@gmail.com](mailto:bauyrzhan.myrzakhmetov86@gmail.com) I would really appreciate your help.