# Applying flat bottom potential on substrate **URL:** \ **Category:** User discussions\ **Tags:** forcefield\ **Created:** [December 28, 2022, 2:06pm UTC](https://gromacs.bioexcel.eu/t/applying-flat-bottom-potential-on-substrate/5500 "2022-12-28T14:06:19Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![l\_aditi](https://avatars.discourse-cdn.com/v4/letter/l/b38774/32.png) [@l\_aditi](https://gromacs.bioexcel.eu/u/l_aditi)\ **Post date:** [December 28, 2022, 2:06pm UTC](https://gromacs.bioexcel.eu/t/applying-flat-bottom-potential-on-substrate/5500/1 "2022-12-28T14:06:19Z")
GROMACS version: 18.8 GROMACS modification: NO I want to apply flat bottom restraint on my substrate while pulling it in the z direction, such that it will able to move freely in a specific area only. Content of file I have created: ; Position restraint for substrate [position\_restraints] ; i funct g r k 8 2 2 0.7 10000 secondly while doing the final umbrella sampling I used this file: ; Position restraint for substrate [position\_restraints] ; i funct g r k 8 2 2 1.0 10000 I wanted to know whether I am mentioning the function and g value correctly or not? what should be the value of funct and g? ---
**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:** [January 13, 2023, 10:10am UTC](https://gromacs.bioexcel.eu/t/applying-flat-bottom-potential-on-substrate/5500/2 "2023-01-13T10:10:57Z")
Hi, It depends what you want to do. In the following links you find the explanation of function values and of the g (geometry) values - a value of funct = 2 in the directive [position\_restrains] correspond to flat bottom restraints see here for [https://manual.gromacs.org/current/reference-manual/topologies/topology-file-formats.html](https://manual.gromacs.org/current/reference-manual/topologies/topology-file-formats.html) - a values of g = 2 In GROMACS 2022, g=2 setting is mapped g=8 see more here [https://manual.gromacs.org/current/reference-manual/functions/restraints.html?highlight=restrain](https://manual.gromacs.org/current/reference-manual/functions/restraints.html?highlight=restrain). I guess you are using an older GROMACS version (series 2018?), in that case good to cross check with the manual of the corresponding version. I hope it helps Alessandra ---
**Author:** ![l\_aditi](https://avatars.discourse-cdn.com/v4/letter/l/b38774/32.png) [@l\_aditi](https://gromacs.bioexcel.eu/u/l_aditi)\ **Post date:** [January 14, 2023, 1:35pm UTC](https://gromacs.bioexcel.eu/t/applying-flat-bottom-potential-on-substrate/5500/3 "2023-01-14T13:35:09Z")
Hello Alessandra, Thanks for clarifying my doubt. Actually, what I am trying to do is pull a substrate through a membrane transporter. For this, I am placing the substrate above the protein (in the solvent) and pulling it using a constant force and velocity. Instead of moving through the transporter pore, it is moving just adjacent to the protein’s surface. I tried to increase the pulling force and velocity, but nothing seems to work. Can you please suggest to me any approach to proceed with?