TIP4P long range solver

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1

Dear lammps-users,

I use TIP4P model of water in my system. For pair style, I’m setting the following;

pair_style hybrid buck/coul/long 8.5 lj/cut/tip4p/long 3 4 2 4 0.1546 8.5

In the doc page for lj/cut/tip4p/long it is written: “style lj/cut/tip4p/long is for use with a long-range Coulombic solver (Ewald or PPPM).”
Is it correct to infer that if I’m using lj/cut/tip4p/long, then “kspace_style ewald 1e-6” will take into account the charge distance from O site in TIP4P?

If so, then why is there a special “pppm/tip4p” to account for long range forces?

I’m using 11Aug17 version of lammps.

Thank you and best regards,

Ali

2

Dear lammps-users,

I use TIP4P model of water in my system. For pair style, I'm setting the
following;

pair_style hybrid buck/coul/long 8.5 lj/cut/tip4p/long 3 4 2 4 0.1546 8.5

In the doc page for lj/cut/tip4p/long it is written: "style
lj/cut/tip4p/long is for use with a long-range Coulombic solver (*Ewald*
or PPPM)."
Is it correct to infer that if I'm using lj/cut/tip4p/long, then
"kspace_style ewald 1e-6" will take into account the charge distance from O
site in TIP4P?

​that is not correct.​

If so, then why is there a special "pppm/tip4p" to account for long range
forces?

​it is there because you must have a modified long-range solver to account
to the charge at a different location than the oxygen location without
explicitly defining its position.

axel.​

3

Dear axel,

Thanks for the response. One more thing:

Does that mean that currently it’s not possible to use ewald summation with TIP4P because we don’t have “ewald/tip4p” option?

Best,
Ali

4

Dear axel,

Thanks for the response. One more thing:

Does that mean that currently it's not possible to use ewald summation
with TIP4P because we don't have "ewald/tip4p" option?

​pppm is equivalent to ewald and usually (much) faster.​

axel.

5

you can also model TIP4P without any of the “tip4p” styles, by defining it through 4 explicit sites and using fix rigid or fix rigid/small instead of fix nve + fix shake to do time integration for the TIP4P water molecules. please search the mailing list archives, this topic has been discussed several times.

axel.

6

It is not currently possible and should error out. I agree the docs are a bit misleading because they mention Ewald in the doc page for lj/cut/tip4p/long.

Stan