# How to pull a group in opposite direction (-Z) in Gromacs?
**URL:** https://gromacs.bioexcel.eu/t/how-to-pull-a-group-in-opposite-direction-z-in-gromacs/1382
**Category:** User discussions
**Created:** [January 5, 2021, 12:08am UTC](https://gromacs.bioexcel.eu/t/how-to-pull-a-group-in-opposite-direction-z-in-gromacs/1382 "2021-01-05T00:08:15Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@Eduardo1](https://gromacs.bioexcel.eu/u/Eduardo1)
#### Post date: [January 5, 2021, 12:08am UTC](https://gromacs.bioexcel.eu/t/how-to-pull-a-group-in-opposite-direction-z-in-gromacs/1382/1 "2021-01-05T00:08:15Z")
Dear Gromacs users,
I have been performing Umbrella Sampling simulations of a host-guest system. Regarding the stage of the steered molecular dynamics simulation, I would like to know how to pull a group along the z coordinate in the opposite direction (x , y , -z).
This is my current pull.mdp file:
; Pull code
pull = yes ; Tells grompp to read settings for COM pulling
pull\_ncoords = 1 ; Only one reaction coordinate
pull\_ngroups = 2 ; Two groups defining one reaction coordinate
pull\_group1\_name = CNH ; Name of the first group used by the pull code.
pull\_group2\_name = CI3 ; Name of the second group used by the pull code.
pull\_coord1\_type = umbrella ; Use harmonic potential to pull
pull\_coord1\_geometry = distance ; Simple distance increase
pull\_coord1\_dim = N N Y ; Applying a bias only in the z-dimension
pull\_coord1\_groups = 1 2 ; The reaction coordinate connects groups 1 and 2.
pull\_coord1\_start = yes ; define initial COM distance \> 0
pull\_coord1\_rate = 0.01 ; 0.01 nm per ps = 10 nm per ns
pull\_coord1\_k = 1000 ; kJ mol^-1 nm^-2
Sincerely,
Eduardo.
---
### Author:  [@yyten](https://gromacs.bioexcel.eu/u/yyten)
#### Post date: [January 5, 2021, 5:44am UTC](https://gromacs.bioexcel.eu/t/how-to-pull-a-group-in-opposite-direction-z-in-gromacs/1382/2 "2021-01-05T05:44:22Z")
My answer could be wrong but isn’t it just a matter of flipping your original structure 180’ so the structure faces the direction you wanted to pull?
I have never done umbrella sampling before, but I did use the technique to modify my protein structure.
---
### Author:  [@Eduardo1](https://gromacs.bioexcel.eu/u/Eduardo1)
#### Post date: [January 5, 2021, 8:56pm UTC](https://gromacs.bioexcel.eu/t/how-to-pull-a-group-in-opposite-direction-z-in-gromacs/1382/3 "2021-01-05T20:56:18Z")
Dear yyten,
Maybe I could do that, but that would involve to rebuild all of my system. Since the force is applied in a unwanted direction, I thought it would be possible to define an opposite direction in the input file for the steered MD simulation. Here is a snapshot of my system:
---
### Author:  [@Harry](https://gromacs.bioexcel.eu/u/Harry)
#### Post date: [May 11, 2021, 8:23am UTC](https://gromacs.bioexcel.eu/t/how-to-pull-a-group-in-opposite-direction-z-in-gromacs/1382/4 "2021-05-11T08:23:48Z")
BS"D
Perhaps you figured this out already, since this was half a year ago, but I think you just specify a negative value for the pull rate.
Harry
---
### Author:  [@Eduardo1](https://gromacs.bioexcel.eu/u/Eduardo1)
#### Post date: [May 11, 2021, 7:18pm UTC](https://gromacs.bioexcel.eu/t/how-to-pull-a-group-in-opposite-direction-z-in-gromacs/1382/5 "2021-05-11T19:18:05Z")
Dear Harry,
Thank you for your response. Actually, I did not find a solution for this question during this time. Thus, I started to conduct these simulations using the Amber software, which I have more experience. Fortunately, I don’t have this problem anymore in this software.
Eduardo.
---
### Author:  [@Yogesh](https://gromacs.bioexcel.eu/u/Yogesh)
#### Post date: [October 23, 2021, 10:07am UTC](https://gromacs.bioexcel.eu/t/how-to-pull-a-group-in-opposite-direction-z-in-gromacs/1382/6 "2021-10-23T10:07:45Z")
hi since gromacs takes Y and N to define x y z pulling. please guide. I am flipping my system 180 degrees. but is there a way to define negative coordinates?
---
### Author:  [@Nishadi](https://gromacs.bioexcel.eu/u/Nishadi)
#### Post date: [October 25, 2021, 5:50am UTC](https://gromacs.bioexcel.eu/t/how-to-pull-a-group-in-opposite-direction-z-in-gromacs/1382/7 "2021-10-25T05:50:47Z")
Dear Eduardo1,
Simply use a negative pull rate. (Reference : [Re: [gmx-users] Steered MD in reverse direction?](https://mail-archive.com/gmx-users@gromacs.org/msg45215.html))
Nishadi
---
### Author:  [@Ranbir720](https://gromacs.bioexcel.eu/u/Ranbir720)
#### Post date: [June 13, 2024, 8:45am UTC](https://gromacs.bioexcel.eu/t/how-to-pull-a-group-in-opposite-direction-z-in-gromacs/1382/8 "2024-06-13T08:45:11Z")
hey , did you find a solution to the issue . Did you try putting a negative pull rate in the pull code