Value of the \sigma parameter for TIP4P water potentially incorrect in charmm36-jul2021.ff

GROMACS version: 2021.5
GROMACS modification: No


I do apologise if this is just me, missing something very obvious, but the value of the LJ \sigma parameter in the ffnonbonded.itp file within the charmm36-jul2021.ff.tgz force field does not look correct to me.

HWT4 1 1.008000 0.000 A 0.000000000000 0.0000000 ; Hydrogen for TIP4P water model
OWT4 8 16.000000 0.000 A 0.648520000000 0.6485200 ; Oxygen for TIP4P water model

In fact, given that it looks identical to the value of the \epsilon parameter, this smells like a copy/paste error to me. I believe the correct value should be 0.3154 (nm).

What’s even more suspicious is the fact that in the older version of the very same force field (e.g. charmm36-feb2021.ff.tgz), the value of \sigma appears to be the correct one instead.

Any hint/comment would be very much appreciated.

Thank you for your help.

Best wishes,


We will look into this and correct it for the future.

Yes, it is definitely a copy-paste error, introduced in the July 2021 release, sorry! The correct value is 0.315365. This is correct the previous release (Feb 2021).