[CP2K-user] [CP2K:19730] Correct TIP3P parameters

[Krack Matthias]

Hi Léon

Yes, 110 kcalmol*rad^-2 and 900 kcal*Angstrom^-2 are correct. Note, that the inputs in cp2k/tests are for code regression testing and the parameters used therein are not reliable by contrast to the example inputs given in the exercises. Unfortunately, many regression tests use this incorrect flexible TIP3P force field. This is most likely a copy-paste error which is indeed confusing for users. I will fix that.

Best

Matthias

From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Léon Luntadila Lufungula <Leon.luntadilalufungula at uantwerpen.be>
Date: Friday, 12 January 2024 at 10:09
To: cp2k <cp2k at googlegroups.com>
Subject: Re: [CP2K:19729] Correct TIP3P parameters
Hi Alin and Matthias,

Thanks for the quick reply, I didn't know that! If so, the 110/900 combination in the water.inp file should be correct then right? Still, in a lot of input files in the tests folder like for example cp2k/tests/Fist/regtest-4/H2O-1.inp (see attachment) the combination 55/450 is used. Can you explain why this is the case then?

Kind regards,
Léon

On Friday 12 January 2024 at 10:02:01 UTC+1 Krack Matthias wrote:
Dear Léon

Note that LAMMPS uses a different potential definition<https://docs.lammps.org/angle_harmonic.html>. By contrast to CP2K, LAMMPS includes the prefactor ½ into the constant K.

HTH

Matthias

From: cp... at googlegroups.com <cp... at googlegroups.com> on behalf of Léon Luntadila Lufungula <Leon.luntad... at uantwerpen.be>
Date: Friday, 12 January 2024 at 09:53
To: cp2k <cp... at googlegroups.com>
Subject: [CP2K:19726] Correct TIP3P parameters
Dear all,

Just a quick question about the parameters for the TIP3P forcefield. In the example files (see attached water.inp) of the exercise<https://www.cp2k.org/exercises:2019_uzh_acpc2:ex02> on molecular simulation of water the TIP3P model is used to simulate water. In the input file, I noticed that the force constant of the H-O-H bend and the H-O bond are 110 rad^-2kcalmol and 900 angstrom^-2kcalmol, respectively. However, when looking online for the parameters like on the lammps documentation<https://docs.lammps.org/Howto_tip3p.html>, I find that these values are actually half of what is defined in the input, i.e., 55 rad^-2kcalmol and 450 angstrom^-2kcalmol, respectively... Is this indeed an error in the input file or am I missing something here? Most water input files I see in the tests/Fist folder also contain the smaller values for the force constants.

Kind regards,
Léon
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