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

**URL:** <https://gromacs.bioexcel.eu/t/value-of-the-sigma-parameter-for-tip4p-water-potentially-incorrect-in-charmm36-jul2021-ff/3736>\
**Category:** User discussions\
**Tags:** forcefield\
**Created:** [February 24, 2022, 11:14am UTC](https://gromacs.bioexcel.eu/t/value-of-the-sigma-parameter-for-tip4p-water-potentially-incorrect-in-charmm36-jul2021-ff/3736 "2022-02-24T11:14:40Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![gsosso](https://avatars.discourse-cdn.com/v4/letter/g/aca169/32.png) [@gsosso](https://gromacs.bioexcel.eu/u/gsosso)\
**Post date:** [February 24, 2022, 11:14am UTC](https://gromacs.bioexcel.eu/t/value-of-the-sigma-parameter-for-tip4p-water-potentially-incorrect-in-charmm36-jul2021-ff/3736/1 "2022-02-24T11:14:40Z")

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GROMACS version: **2021.5**  
GROMACS modification: No

Hello,

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](http://mackerell.umaryland.edu/download.php?filename=CHARMM_ff_params_files/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](http://mackerell.umaryland.edu/download.php?filename=CHARMM_ff_params_files/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,

Gabriele

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**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:** [March 3, 2022, 1:27pm UTC](https://gromacs.bioexcel.eu/t/value-of-the-sigma-parameter-for-tip4p-water-potentially-incorrect-in-charmm36-jul2021-ff/3736/2 "2022-03-03T13:27:30Z")

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

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**Author:** ![awacha](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/awacha/32/19_2.png) [@awacha](https://gromacs.bioexcel.eu/u/awacha)\
**Post date:** [March 3, 2022, 2:35pm UTC](https://gromacs.bioexcel.eu/t/value-of-the-sigma-parameter-for-tip4p-water-potentially-incorrect-in-charmm36-jul2021-ff/3736/3 "2022-03-03T14:35:22Z")

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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).