# Umbrella sampling grompp.cpp error **URL:** \ **Category:** User discussions\ **Tags:** grompp, umbrella-sampling\ **Created:** [September 26, 2022, 10:54am UTC](https://gromacs.bioexcel.eu/t/umbrella-sampling-grompp-cpp-error/4872 "2022-09-26T10:54:21Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![parul](https://avatars.discourse-cdn.com/v4/letter/p/5fc32e/32.png) [@parul](https://gromacs.bioexcel.eu/u/parul)\ **Post date:** [September 26, 2022, 10:54am UTC](https://gromacs.bioexcel.eu/t/umbrella-sampling-grompp-cpp-error/4872/1 "2022-09-26T10:54:21Z")
GROMACS version:2021.4 GROMACS modification: no gmx grompp -f npt\_umbrella.mdp -c conf449.gro -p topol.top -r conf449.gro -n index.ndx -o npt22.tpr -maxwarn 1 when i tried to run this command in umbrella sampling tutorial ,its giving the following error: NOTE 3 [file npt\_umbrella.mdp]: Removing center of mass motion in the presence of position restraints might cause artifacts. When you are using position restraints to equilibrate a macro-molecule, the artifacts are usually negligible. Calculating fourier grid dimensions for X Y Z Using a fourier grid of 56x40x104, spacing 0.118 0.110 0.116 ERROR 1 [file npt\_umbrella.mdp]: When the maximum distance from a pull group reference atom to other atoms in the group is larger than 0.5 times half the box size a centrally placed atom should be chosen as pbcatom. Pull group 1 is larger than that and does not have a specific atom selected as reference atom. Pull group natoms pbc atom distance at start reference at t=0 1 226 113 2 226 339 4.868 nm 4.868 nm Estimate for the relative computational load of the PME mesh part: 0.10 This run will generate roughly 13 Mb of data There were 3 notes There was 1 warning * * * Program: gmx grompp, version 2021.4-Ubuntu-2021.4-2 Source file: src/gromacs/gmxpreprocess/grompp.cpp (line 2389) Fatal error: There was 1 error in input file(s) I don’t know why this error is coming as the input file npt\_umbrella.mdp , I downloaded from the gromacs tutorial itself, I am not able to un derstand why this error is coming,what does this error mean? kindly help me with it . ---
**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:** [September 27, 2022, 7:47am UTC](https://gromacs.bioexcel.eu/t/umbrella-sampling-grompp-cpp-error/4872/2 "2022-09-27T07:47:10Z")
Hi, I guess the error indicates that pull-group1-pbcatom option in the mdp file should be defined differently ([Molecular dynamics parameters (.mdp options) — GROMACS 2022.3 documentation](https://manual.gromacs.org/current/user-guide/mdp-options.html#mdp-pull-group1-pbcatom)). This option is relevant when you deal with relative large group. \Alessandra ---
**Author:** ![parul](https://avatars.discourse-cdn.com/v4/letter/p/5fc32e/32.png) [@parul](https://gromacs.bioexcel.eu/u/parul)\ **Post date:** [September 29, 2022, 5:42am UTC](https://gromacs.bioexcel.eu/t/umbrella-sampling-grompp-cpp-error/4872/3 "2022-09-29T05:42:44Z")
thank you alevilla it worked