# Mdp file differences between CHARMM27 and CHARMM36 parameters **URL:** \ **Category:** User discussions\ **Tags:** forcefield, mdp-parameters, simulation-setup\ **Created:** [October 4, 2026, 6:11am UTC](https://gromacs.bioexcel.eu/t/mdp-file-differences-between-charmm27-and-charmm36-parameters/13747 "2026-10-04T06:11:06Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![ishanipaul](https://avatars.discourse-cdn.com/v4/letter/i/f08c70/32.png) [@ishanipaul](https://gromacs.bioexcel.eu/u/ishanipaul)\ **Post date:** [October 4, 2026, 6:11am UTC](https://gromacs.bioexcel.eu/t/mdp-file-differences-between-charmm27-and-charmm36-parameters/13747/1 "2026-10-04T06:11:06Z")
GROMACS version: 2024.5 GROMACS modification: No The new version of GROMACS documentation suggests the following modification in the md.mdp file for CHARMM36 forcefield, constraints = h-bonds cutoff-scheme = Verlet vdwtype = cutoff vdw-modifier = force-switch rlist = 1.2 rvdw = 1.2 rvdw-switch = 1.0 coulombtype = PME rcoulomb = 1.2 DispCorr = no But I could not find the updated version and used the following parameters for my simulation: title = OPLS Lysozyme NPT equilibration ; Run parameters integrator = md ; leap-frog integrator nsteps = 500000000 ; 2 \* 500000000 = 1000000 ps (1000 ns) dt = 0.002 ; 2 fs ; Output control nstxout = 0 ; suppress bulky .trr file by specifying nstvout = 0 ; 0 for output frequency of nstxout, nstfout = 0 ; nstvout, and nstfout nstenergy = 50000 ; save energies every 100.0 ps nstlog = 50000 ; update log file every 100.0 ps nstxout-compressed = 50000 ; save compressed coordinates every 100.0 ps compressed-x-grps = System ; save the whole system ; Bond parameters continuation = yes ; Restarting after NPT constraint\_algorithm = lincs ; holonomic constraints constraints = h-bonds ; bonds involving H are constrained lincs\_iter = 1 ; accuracy of LINCS lincs\_order = 4 ; also related to accuracy ; Neighborsearching cutoff-scheme = Verlet ; Buffered neighbor searching ns\_type = grid ; search neighboring grid cells nstlist = 10 ; 20 fs, largely irrelevant with Verlet scheme rcoulomb = 1.0 ; short-range electrostatic cutoff (in nm) rvdw = 1.0 ; short-range van der Waals cutoff (in nm) ; Electrostatics coulombtype = PME ; Particle Mesh Ewald for long-range electrostatics pme\_order = 4 ; cubic interpolation fourierspacing = 0.16 ; grid spacing for FFT ; Temperature coupling is on tcoupl = V-rescale ; modified Berendsen thermostat tc-grps = Protein Non-Protein ; two coupling groups - more accurate tau\_t = 0.1 0.1 ; time constant, in ps ref\_t = 310 310 ; reference temperature, one for each group, in K ; Pressure coupling is on pcoupl = Parrinello-Rahman ; Pressure coupling on in NPT pcoupltype = isotropic ; uniform scaling of box vectors tau\_p = 2.0 ; time constant, in ps ref\_p = 1.0 ; reference pressure, in bar compressibility = 4.5e-5 ; isothermal compressibility of water, bar^-1 ; Periodic boundary conditions pbc = xyz ; 3-D PBC ; Dispersion correction DispCorr = EnerPres ; account for cut-off vdW scheme ; Velocity generation gen\_vel = no ; Velocity generation is off What could be the possible implications and how could it affect my results? ---
**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\ **Post date:** [October 4, 2026, 10:07pm UTC](https://gromacs.bioexcel.eu/t/mdp-file-differences-between-charmm27-and-charmm36-parameters/13747/2 "2026-10-04T22:07:46Z")
Nonbonded settings are an integral part of the force field. You should use what is recommended for your chosen force field. You’re using conventional OPLS settings for CHARMM, which is wholly inappropriate. ---
**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:** [October 5, 2026, 8:31am UTC](https://gromacs.bioexcel.eu/t/mdp-file-differences-between-charmm27-and-charmm36-parameters/13747/3 "2026-10-05T08:31:00Z")
In addition we recommend using a single temperature coupling group and tau=1 ps, as well as the c-rescale barostat with tau\_p=5 ps. Using Parrinello-Rahman with a period of 2 ps can cause instabilities. ---
**Author:** ![ishanipaul](https://avatars.discourse-cdn.com/v4/letter/i/f08c70/32.png) [@ishanipaul](https://gromacs.bioexcel.eu/u/ishanipaul)\ **Post date:** [October 7, 2026, 10:57am UTC](https://gromacs.bioexcel.eu/t/mdp-file-differences-between-charmm27-and-charmm36-parameters/13747/4 "2026-10-07T10:57:49Z")
Thank you for your response. Can anyone share a standard CHARMM27-supported md.mdp file here so that I could understand the basic parameter setting for a specific forcefield? ---
**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\ **Post date:** [October 7, 2026, 12:01pm UTC](https://gromacs.bioexcel.eu/t/mdp-file-differences-between-charmm27-and-charmm36-parameters/13747/5 "2026-10-07T12:01:23Z")
The files found at [Lysozyme in Water](http://www.mdtutorials.com/gmx/lysozyme/index.html) are correct for use with the CHARMM force field (note that the protein force field is not called “CHARMM27” it is CHARMM22/CMAP) and the latest version is CHARMM36m. I would strongly recommend the latest force field unless you have a compelling scientific reason to use a force field that is over 20 years old.