# PULL CODE: applying force
**URL:** https://gromacs.bioexcel.eu/t/pull-code-applying-force/3000
**Category:** User discussions
**Created:** [October 21, 2021, 3:51am UTC](https://gromacs.bioexcel.eu/t/pull-code-applying-force/3000 "2021-10-21T03:51:28Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@KSY](https://gromacs.bioexcel.eu/u/KSY)
#### Post date: [October 21, 2021, 3:51am UTC](https://gromacs.bioexcel.eu/t/pull-code-applying-force/3000/1 "2021-10-21T03:51:28Z")
GROMACS version: 2020.1
GROMACS modification: Yes/No
In order to apply the periodically changed force, which option should I use in the pull-coord1-type?
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### Author:  [@hess](https://gromacs.bioexcel.eu/u/hess)
#### Post date: [October 26, 2021, 8:10am UTC](https://gromacs.bioexcel.eu/t/pull-code-applying-force/3000/2 "2021-10-26T08:10:25Z")
I don’t understand exactly what you mean with “periodically changed force”. You can continuously change the reference distance with pull-coord1-rate.
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### Author:  [@KSY](https://gromacs.bioexcel.eu/u/KSY)
#### Post date: [October 27, 2021, 5:22am UTC](https://gromacs.bioexcel.eu/t/pull-code-applying-force/3000/3 "2021-10-27T05:22:55Z")
Thank you for your reply. I mean the external force like vibration. Is it possible to pull molecule in back and forth?
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### Author:  [@hess](https://gromacs.bioexcel.eu/u/hess)
#### Post date: [October 27, 2021, 10:11am UTC](https://gromacs.bioexcel.eu/t/pull-code-applying-force/3000/4 "2021-10-27T10:11:51Z")
No, this is not possible. It is not difficult to change this in the code though.