[lammps-users] Lammps Featured on Podcast

Thank you very much to Steve Plimpton for taking an hour to speak with me (Brock Palen) and Jeff Squyres about lammps.

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Brock Palen
www.umich.edu/~brockp
Center for Advanced Computing
[email protected]
(734)936-1985

I'm simulating tip4p in a slab configuration under an electric filed. I'm getting funny results for the dielectric constant. It just occurred to me that "kspace_modify slab" might not be implemented in the tip4p_pppm. I'm a C++ novice, so it's difficult for me to tell, but it looks as if tip4p_pppm doesn't call the slab correction from the pppm routine. Can someone verify this for me?
Thanks,
Matt

Matt:

The TIP4P routine is a derived class of the generic PPPM; so basically, the only routines that have changed from PPPM to PPPM/TIP4P are the four routines in TIP4P, none of which involve the slab correction explicitly. (The slab correction is passed into those routines through other parameters, however, so it is taken into account.)

–AEI

Hi Ahmed. Along these same lines - I tried using fix efield to double check the results I'm getting from two parallel charged plates. I don't think the fix adds the forces correctly. It seems to be adding the force as if the charge was on the tip4p oxygen. Again, I'm not up to speed with C++ so I can't really be certain this is so.
Matt

Quoting "Ahmed E. Ismail" <[email protected]>:

Matt:

You are correct–fix efield will not consider the TIP4P charges, because it doesn’t know how to. TIP4P support is only available in the pair and kspace modules. If you want to use efield (or other routines) that compute forces, you’ll need to add support for TIP4P on a module-by-module basis. (The routines to look at are the differences between pppm and pppm_tip4p.cpp; you’ll see that there’s not a lot of code that has to be changed. Basically, you proceed as normal unless the atom is a TIP4P oxygen, in which case you execute the force calculations and redistributions that are outlined in pppm_tip4p.cpp (or pair_lj_cut_coul_long_tip4p.cpp).

–AEI

Fix efield is very simple. It just adds a qE force to each atom
in the group, where q is the atom's charge and E is the value
you input. If that's not valid for a TIP4P water, e.g. you want
to add force to the pseudo-charge and thus indirectly to
all 3 atoms, then you'd have to add logic to fix efield to do that.

Steve

OK. Depending on the amount of time I have and how married the coauthors are to TIP4P I might implement this. If I do I'll pass it on.
Matt

Quoting Steve Plimpton <[email protected]>:

Hi Steve. The online LAMMPS page for fix efield has a broken link. The link to the Variable page points to http://lammps.sandia.gov/doc/variable -it needs to have .html appended.
Thanks,
Matt

Fixed it - thanks

Steve