# Gromacs-CP2k energy minimization

**URL:** https://gromacs.bioexcel.eu/t/gromacs-cp2k-energy-minimization/13241
**Category:** User discussions
**Tags:** mdrun
**Created:** [February 24, 2026, 12:08pm UTC](https://gromacs.bioexcel.eu/t/gromacs-cp2k-energy-minimization/13241 "2026-02-24T12:08:43Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![krsiwa](https://avatars.discourse-cdn.com/v4/letter/k/77aa72/32.png) [@krsiwa](https://gromacs.bioexcel.eu/u/krsiwa)
#### Post date: [February 24, 2026, 12:08pm UTC](https://gromacs.bioexcel.eu/t/gromacs-cp2k-energy-minimization/13241/1 "2026-02-24T12:08:43Z")

</div>

Hi everyone,

I am trying to perform a QM/MM geometry optimization after an MD simulation using the Gromacs-Cp2k interface. I would like to use the L-BFGS integrator for minimization, but I receive the following error:

```auto
Fatal error:

The combination of constraints and L-BFGS minimization is not implemented."

Either do not use constraints or use a different minimizer (e.g., steepest descent).

```

I understand that this algorithm is not implemented with constraints. Even when I set constraints = none, I get this error. How can I overcome this error?

Any suggestions are highly appreciated. Thank you in advance.