# Comparing Interaction Energy Analysis: QM vs. Force Field Approaches

**URL:** <https://gromacs.bioexcel.eu/t/comparing-interaction-energy-analysis-qm-vs-force-field-approaches/9462>\
**Category:** User discussions\
**Tags:** forcefield, mdp-parameters, mdrun, analysis-tools, simulation-setup\
**Created:** [June 21, 2024, 12:33am UTC](https://gromacs.bioexcel.eu/t/comparing-interaction-energy-analysis-qm-vs-force-field-approaches/9462 "2024-06-21T00:33:27Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![ealanis](https://avatars.discourse-cdn.com/v4/letter/e/e99b99/32.png) [@ealanis](https://gromacs.bioexcel.eu/u/ealanis)\
**Post date:** [June 21, 2024, 12:33am UTC](https://gromacs.bioexcel.eu/t/comparing-interaction-energy-analysis-qm-vs-force-field-approaches/9462/1 "2024-06-21T00:33:27Z")

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How can one perform an interaction energy (IE) analysis using GROMACS to compare with QM data? For example, consider the interaction energy curve of a cation with a molecule like methanol. QM calculations are conducted in a vacuum at 0 K, where the interaction energy (IE) is computed as: IE = E(dimer) - E(fragment1) - E(fragment2).

For the force-field part, one could emulate a vacuum calculation by using a very large simulation box and large cutoffs. Then, vary the cation-ligand distance in a 0-step molecular dynamics run (mdrun) for each distance step.

Are these results comparable? How so? How does one calculate the interaction energy using the force-field approach?

I’ve seen this type of curves in e.g. [https://pubs.acs.org/doi/epdf/10.1021/acs.jpcb.2c07237](https://pubs.acs.org/doi/epdf/10.1021/acs.jpcb.2c07237).

 ![QM-MM-IC](https://europe1.discourse-cdn.com/flex017/uploads/bioexcel1/original/2X/6/699fce98543a34d53670c250568423a2c6579226.jpeg)

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**Author:** ![milosz.wieczor](https://avatars.discourse-cdn.com/v4/letter/m/b19c9b/32.png) [@milosz.wieczor](https://gromacs.bioexcel.eu/u/milosz.wieczor)\
**Post date:** [June 21, 2024, 2:31pm UTC](https://gromacs.bioexcel.eu/t/comparing-interaction-energy-analysis-qm-vs-force-field-approaches/9462/2 "2024-06-21T14:31:08Z")

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In general, `gmx energy` does this. It’s what this Gromologist workflow uses under the hood to make things simpler:

> **[Performing interaction energy decomposition · Wiki · KBM / Gromologist · GitLab](https://gitlab.com/KomBioMol/gromologist/-/wikis/Performing-interaction-energy-decomposition)**
>
> Package to handle GROMACS topology stuff

Whether they are comparable in practice - they should be, but it might depend on what the target of the original parameterization was. Some models are fine-tuned to reproduce energetics in vacuo, some to reproduce free energy profiles in water (or some combination of these two), and some to reproduce average molecular properties.