# Calculate FES of Slow-Growth FEP

**URL:** <https://gromacs.bioexcel.eu/t/calculate-fes-of-slow-growth-fep/722>\
**Category:** User discussions\
**Created:** [August 28, 2020, 11:54am UTC](https://gromacs.bioexcel.eu/t/calculate-fes-of-slow-growth-fep/722 "2020-08-28T11:54:22Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![vg87](https://avatars.discourse-cdn.com/v4/letter/v/65b543/32.png) [@vg87](https://gromacs.bioexcel.eu/u/vg87)\
**Post date:** [August 28, 2020, 11:54am UTC](https://gromacs.bioexcel.eu/t/calculate-fes-of-slow-growth-fep/722/1 "2020-08-28T11:54:22Z")

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GROMACS version: 5.1.4  
GROMACS modification: No  
Hi all,

I am writing you cause I am in trouble and not sure about the correctness of my results.  
Briefly, I set up a free-energy perturbation with the slow-growth approach. Hence, lambda value slightly increase at each MD step.  
I have 10 different MDs  
Here the free-energy section of my fep.mdp file:

; Free energy control stuff  
free\_energy = yes  
init\_lambda = 0  
delta\_lambda = 0.00000004  
sc-alpha = 0  
sc-sigma = 0.3

Of course TypeB section in the topology is modified accordingly.  
Calculations goes fine and at the end i get a file named ‘fep.xvg’ containing dH/dLambda along the frames. So far clear.  
However, I have a doubt about the numerical integration step to obtain the free energy going from lambda=0 to lambda=1  
I have tried gmx bar as follow:

**gmx bar -f fep.xvg (w/wo -g fep.edr)** getting the following error:

“Fatal error:  
While trying to read the lambda value from the file path: last directory in the path ‘fep.xvg’ does not contain a number”  
I suppose such an error arises because gmx bar expect a fixed lambda value while here I have used a slow-growth approach.

Do you have any ideas/experiences/suggestions on the best way to better extrapolate FES?

Thanks in advance for any kind contribution.

Regards

VG

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<div class="post-metadata">

**Author:** ![vg87](https://avatars.discourse-cdn.com/v4/letter/v/65b543/32.png) [@vg87](https://gromacs.bioexcel.eu/u/vg87)\
**Post date:** [September 1, 2020, 8:49am UTC](https://gromacs.bioexcel.eu/t/calculate-fes-of-slow-growth-fep/722/2 "2020-09-01T08:49:18Z")

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No ideas?

Thanks

V