# Free energy perturbation for mass and constraints are not supported

**URL:** <https://gromacs.bioexcel.eu/t/free-energy-perturbation-for-mass-and-constraints-are-not-supported/1700>\
**Category:** User discussions\
**Created:** [March 7, 2021, 3:17pm UTC](https://gromacs.bioexcel.eu/t/free-energy-perturbation-for-mass-and-constraints-are-not-supported/1700 "2021-03-07T15:17:32Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![kexul](https://avatars.discourse-cdn.com/v4/letter/k/73ab20/32.png) [@kexul](https://gromacs.bioexcel.eu/u/kexul)\
**Post date:** [March 7, 2021, 3:17pm UTC](https://gromacs.bioexcel.eu/t/free-energy-perturbation-for-mass-and-constraints-are-not-supported/1700/1 "2021-03-07T15:17:33Z")

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```auto
GROMACS version: 2021
GROMACS modification: No

```

Hi, when I set `-update gpu` with `mdrun`, an error raised:

```auto
Inconsistency in user input:
Update task on the GPU was required,
but the following condition(s) were not satisfied:
Free energy perturbation for mass and constraints are not supported.

```

Here is my `.mdp` file, I think I’ve set the mass and restrained lambda to zero, why did this error raised?

[Production\_158.mdp](https://gromacs.bioexcel.eu/uploads/short-url/hXpvIgdUKa36eEboTjG7TnAgF2y.mdp) (4.6 KB)

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**Author:** ![kexul](https://avatars.discourse-cdn.com/v4/letter/k/73ab20/32.png) [@kexul](https://gromacs.bioexcel.eu/u/kexul)\
**Post date:** [June 24, 2021, 7:42am UTC](https://gromacs.bioexcel.eu/t/free-energy-perturbation-for-mass-and-constraints-are-not-supported/1700/2 "2021-06-24T07:42:00Z")

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Can anyone help? here is the files I used.  
[morph.top](https://gromacs.bioexcel.eu/uploads/short-url/jbhekDqpmFNV3wnTZnM7GmP6yAh.top) (35.3 KB)  
[Production\_0.mdp](https://gromacs.bioexcel.eu/uploads/short-url/lp1Vb8wEe0eQFborsyHyntTNmVy.mdp) (1.7 KB)

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**Author:** ![alevilla](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/alevilla/32/439_2.png) [@alevilla](https://gromacs.bioexcel.eu/u/alevilla)\
**Post date:** [June 24, 2021, 11:27am UTC](https://gromacs.bioexcel.eu/t/free-energy-perturbation-for-mass-and-constraints-are-not-supported/1700/3 "2021-06-24T11:27:29Z")

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Hi,  
In the topology you perturb Carbon mass to hydrogen mass,  
`mass-lambdas` = 0 0 etc is assigning a lambda value = zero.  
I guess that it is the reason of the error. You can try to run on CPU.  
Best regards  
Alessandra

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<div class="post-metadata">

**Author:** ![kexul](https://avatars.discourse-cdn.com/v4/letter/k/73ab20/32.png) [@kexul](https://gromacs.bioexcel.eu/u/kexul)\
**Post date:** [June 25, 2021, 9:28am UTC](https://gromacs.bioexcel.eu/t/free-energy-perturbation-for-mass-and-constraints-are-not-supported/1700/4 "2021-06-25T09:28:24Z")

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Thanks @alevilla