Question on TIP4P Parameters

LAMMPS LAMMPS General Discussion
1

Hi all,

I have some questions about the forcefield parameters for the TIP4P models in the LAMMPS documentations, mainly the sigma values for the hydrogen atoms.

For the current LAMMPS documentation the sigma values for hydrogen interactions are suggested as 1.0. (8.4.4. TIP4P water model — LAMMPS documentation)

However, in the old documentation they are stated to be 0.0. (https://www.afs.enea.it/software/lammps/doc19/html/Howto_tip4p.html)

By the definition in the TIP4P-2005 paper (In TIP4P there is a single Lennard-Jones (LJ) interaction site at the oxygen and electrostatic charges at the hydrogens. H and M sites are charged, but do not contribute to the LJ term), it seems like the hydrogen atoms should have a sigma value of 0.0 when using geometric mixing rules. But then this seems conflicting to the current LAMMPS TIP4P how-to. I’m wondering which is the correct sigma value for the hydrogen atoms / does it depend on the version of LAMMPS used?

Thanks,
Charlotte

2

The value of sigma is irrelevant because the value of epsilon is zero.
Using sigma == 0 can produce problems/crashes in some rare cases where it would trigger a division by zero. This is avoided by setting sigma to 1.0.

3

Ah, thank you for the clarification!