# Box volume expansion at MD

**URL:** <https://gromacs.bioexcel.eu/t/box-volume-expansion-at-md/4119>\
**Category:** User discussions\
**Created:** [May 6, 2022, 1:29pm UTC](https://gromacs.bioexcel.eu/t/box-volume-expansion-at-md/4119 "2022-05-06T13:29:59Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![XiaoLei](https://avatars.discourse-cdn.com/v4/letter/x/71c47a/32.png) [@XiaoLei](https://gromacs.bioexcel.eu/u/XiaoLei)\
**Post date:** [May 6, 2022, 1:30pm UTC](https://gromacs.bioexcel.eu/t/box-volume-expansion-at-md/4119/1 "2022-05-06T13:30:00Z")

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GROMACS version:2020.6  
GROMACS modification: No  
When I do MD simulation about DES extraction citral, the volume of the box becomes huge after a period of time.And before MD simulation, NVT and NPT has been finished normally, so anyone can tell how to solve this problem?  
here is my md.mdp：  
define =  
integrator = md  
dt = 0.001 ; ps  
nsteps = 100000000 ; 200ns  
comm-grps = system  
energygrps =  
;  
nstxout = 2000  
nstvout = 2000  
nstfout = 2000  
nstlog = 2000  
nstenergy = 2000  
nstxout-compressed = 2000  
compressed-x-grps = system  
;  
pbc = xyz  
cutoff-scheme = Verlet  
coulombtype = PME  
rcoulomb = 1.2  
vdwtype = cut-off  
rvdw = 1.2  
DispCorr = EnerPres  
;  
Tcoupl = V-rescale  
tau\_t = 0.2  
tc\_grps = system  
ref\_t = 338.15  
;  
Pcoupl = parrinello-rahman  
pcoupltype = isotropic  
tau\_p = 2.0  
ref\_p = 1.0  
compressibility = 4.5e-5  
;  
gen\_vel = no  
gen\_temp = 298.15  
gen\_seed = -1  
;

 ![sendpix0](https://europe1.discourse-cdn.com/flex017/uploads/bioexcel1/original/2X/f/f30c3183a1500084ae881deae6f58ed180b252b4.jpeg)

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<div class="post-metadata">

**Author:** ![hess](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/hess/32/416_2.png) [@hess](https://gromacs.bioexcel.eu/u/hess)\
**Post date:** [May 11, 2022, 8:04pm UTC](https://gromacs.bioexcel.eu/t/box-volume-expansion-at-md/4119/2 "2022-05-11T20:04:16Z")

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The period of your PR barostat is 2 ps, which is very short. Likely this couples to a vibrational mode in your system. Increase tau\_p or user a newer version of Gromacs and the c-rescale barostat.