# Fast NPT equilibration **URL:** \ **Category:** User discussions\ **Created:** [September 14, 2025, 4:25pm UTC](https://gromacs.bioexcel.eu/t/fast-npt-equilibration/12640 "2025-09-14T16:25:50Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![penelope](https://avatars.discourse-cdn.com/v4/letter/p/53a042/32.png) [@penelope](https://gromacs.bioexcel.eu/u/penelope)\ **Post date:** [September 14, 2025, 4:25pm UTC](https://gromacs.bioexcel.eu/t/fast-npt-equilibration/12640/1 "2025-09-14T16:25:50Z")
GROMACS version: 2021.4 GROMACS modification: Yes/No Here post your question: I’ve created a simulation box including 1128 EC solvents, 37 magnesiums, 74 TFSI anions and 16 12-crown-4 in a 5\*5\*5 nm³ simulation box applying OPLSAA FF. However, after 10 ns of NPT equilibration, the GRO/coordinate file reveals that my simulation box expanded from 5 nm to 5.3 nm in three directions. After generating the density graph, the initial density is \>1600 kg/m³, the average density is 1400 kg/m³, and the density of my EC solvent is 1320 kg/m³. However, I started my pressure at 1 atm, but after equilibration, the pressure ended at 4 bar. I think my equilibration is too fast, and I really don’t know how I can control the pressure of my system. I also want to know if the expansion of my box is fine. Should I proceed with annealing based on the size of this box, or do I need to make it 5 nm in all three directions, as I initially specified for the box size? Can anyone please help me know where I’m going wrong? I’m new to this. npt.mdp is as follows: ; Run parameters integrator = md nsteps = 10000000 ; dt = 0.001 ; 1 fs ; Output control nstxout-compressed = 10000 ; Save .xtc every 10 ps nstxout = 1000 ; Save coordinates every 1 ps nstvout = 1000 ; Save velocities every 1 ps nstenergy = 1000 ; Save energies every 1 ps nstlog = 1000 ; Update log every 1 ps ; Bond parameters constraint\_algorithm = lincs constraints = none lincs\_iter = 1 ; accuracy of LINCS lincs\_order = 4 ; also related to accuracy ; Neighbor searching and vdW cutoff-scheme = Verlet nstlist = 10 ; largely irrelevant with Verlet ns\_type = grid rlist = 1.2 rvdw = 1 ; short-range van der Waals cutoff (in nm) vdw-type = Cut-off ; Electrostatics coulombtype = pme rcoulomb = 1.2 ; Ewald fourierspacing = 0.12 fourier\_nx = 0 fourier\_ny = 0 fourier\_nz = 0 ewald\_rtol = 1e-05 epsilon\_surface = 0 pme-order = 4 ; Temperature coupling tcoupl = v-rescale tc-grps = System tau\_t = 1 ref\_t = 303K ; Pressure coupling pcoupl = Berendsen pcoupltype = isotropic tau\_p = 5.0 ; time constant, in ps ref\_p = 1.0 ; 1 atm compressibility = 4.5e-5 ; Periodic boundary conditions pbc = xyz ; Dispersion correction DispCorr = EnerPres ; Velocity generation gen\_vel = no ---
**Author:** ![lmullender](https://avatars.discourse-cdn.com/v4/letter/l/8dc957/32.png) [@lmullender](https://gromacs.bioexcel.eu/u/lmullender)\ **Post date:** [September 16, 2025, 12:00pm UTC](https://gromacs.bioexcel.eu/t/fast-npt-equilibration/12640/2 "2025-09-16T12:00:34Z")
Hi @penelope, Pressure coupling in molecular simulations works by changing the size of the simulation box, so the change you are seeing is entirely expected. Also, the instantaneous pressure is expected to fluctuate by quite a bit, so it’s important to check the average values rather than the ones the simulation ended on. Looking at your mdp settings, you’re using the Berendsen barostat, which is outdated and should not be used. I would try c-rescale instead. Also, 4.5e-5 bar^-1 is the compressibility of water, so if you’re using a different solvent you should set the appropriate value for the compressibility. ---
**Author:** ![penelope](https://avatars.discourse-cdn.com/v4/letter/p/53a042/32.png) [@penelope](https://gromacs.bioexcel.eu/u/penelope)\ **Post date:** [September 16, 2025, 12:24pm UTC](https://gromacs.bioexcel.eu/t/fast-npt-equilibration/12640/3 "2025-09-16T12:24:40Z")
Hello Imullender, Thank you very much for the reply.