# OH bond force constant in spc/spce water model

**URL:** <https://gromacs.bioexcel.eu/t/oh-bond-force-constant-in-spc-spce-water-model/7322>\
**Category:** User discussions\
**Tags:** forcefield\
**Created:** [October 3, 2023, 1:39pm UTC](https://gromacs.bioexcel.eu/t/oh-bond-force-constant-in-spc-spce-water-model/7322 "2023-10-03T13:39:01Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![lige1018](https://avatars.discourse-cdn.com/v4/letter/l/cab0a1/32.png) [@lige1018](https://gromacs.bioexcel.eu/u/lige1018)\
**Post date:** [October 3, 2023, 1:39pm UTC](https://gromacs.bioexcel.eu/t/oh-bond-force-constant-in-spc-spce-water-model/7322/1 "2023-10-03T13:39:01Z")

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Hi, I noticed that in the spc.itp and spce.itp of GROMACS, the OH bond force constant is 345000 kj/(mol nm2). However, in LAMMPS, this constant is 553.935 kcal/(mol A2). If it is converted to GROMACS unit, it will be around 463533 kj/(mol nm2). I am wondering why there is a difference between GROMACS and LAMMPS? Thanks!

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**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 3, 2023, 3:11pm UTC](https://gromacs.bioexcel.eu/t/oh-bond-force-constant-in-spc-spce-water-model/7322/2 "2023-10-03T15:11:07Z")

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SPC and SPC/E models have no bond force constants as the models are rigid. The only reason to have a bond potentials in files for those models is to allow methods that do not support constraints to work, in particular energy minimization.

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**Author:** ![lige1018](https://avatars.discourse-cdn.com/v4/letter/l/cab0a1/32.png) [@lige1018](https://gromacs.bioexcel.eu/u/lige1018)\
**Post date:** [October 4, 2023, 9:07am UTC](https://gromacs.bioexcel.eu/t/oh-bond-force-constant-in-spc-spce-water-model/7322/3 "2023-10-04T09:07:20Z")

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Thanks. Is it the same for the angle parameters below the bond parameters that they only function if water molecules are defined as flexible?

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**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 4, 2023, 10:05am UTC](https://gromacs.bioexcel.eu/t/oh-bond-force-constant-in-spc-spce-water-model/7322/4 "2023-10-04T10:05:14Z")

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Indeed.