[Pw_forum] MD simulations of polar liquids

[Nichols A. Romero]

Thanks to Paul Tangney for a very comprehensive and useful explanation. I
believe that you are absolute right wrt
to the issues to the electrostatic issues that arise in DFT vs. classical MD
calculations.

I would like to expand upon the issue that I am running into in the hopes
that somebody on the list
may have an idea.

I am studying liquid nitromethane in a VERY small unit cell. There are only
8 nitromethane molecules.
Already a problematic systems because the predominat interaction is vdW. It
is also at a very low density.
I run an NVT simulation for about 8 ps. I wrote a seperate program that
takes each nitromethane molecular
and calculation the translational kinetic energy, rotation kinetic energy of
the molecular (I recalculate
the moment of inertia at each step) and outputs a temperature based on those
quantities, e.g. T_trans=KE_trans/(3*N_mol)

Anyhow, if the system is properly equilibrated, I would think that the
average T_trans and T_rot would be equal
to the imposed temperature by the thermostat. However, I am finding that it
is lower by 20-40% lower. My only
ideas are that:
1. That this system requires ALOT more time to equilibrate.
2. That initially all the energy must be getting locked into intramolecular
vibrations.

Ideas anyone?

On 1/4/07, Paul Tangney <tangney at civet.berkeley.edu> wrote:
>
>
> Hi Nichols,
>
> Dipole corrections to what quantity ?
>
> There is now a large literature on macroscopic polarization
> in the context of DFT (Resta, Vanderbilt and others) but if
> I understand your problem correctly, it is not necessary for
> you to delve into this.
>
> Slabs with net dipole moments under periodic boundary
> conditions are problematic because, in a slab calculation,
> you don't generally want the periodic images to 'see' each other, but
> a slab with a strong moment interacts with its images.
> If a dipole approximation is valid (if the spatial separation
> of charges is small compared to your unit cell) the dipole-dipole
> interaction decays with 1/r^5.
>
> For a bulk liquid, the problem should be much less serious because
> it is less ordered and there is no vacuum.
> There shouldn't be any strong multipole moments and any moments that
> do exist should be well screened and transient.
>
> Finite size effects are *much* *much* more serious for simple
> classical MD - particularly when only point-charge electrostatics
> are included. The reason is that, even in strongly ionic materials
> such as NaCl, water or silica, very simple electronic screening mechanisms
> (such as screening from polarizable atoms) are sufficient
> to kill electrostatic interactions within a few nanometers..and usually
> within 1 nm. On the other hand, finite size effects in MD with simple
> classical potentials are large because the only screening is from
> the ions themselves and occurs on ionic time scales.
> As a result, some physical properties (e.g. thermal expansion) require
> 1000 to 10000 atoms for convergence while the same property with
> DFT is converged with 50 to 100 atoms.
>
> I have been waving my hands while typing this - I'm going by
> my experience but these issues are poorly controlled, and
> should be investigated more thoroughly.
> Hopefully somebody will have the time some day.
>
> Best regards,
>
> Paul
>
> > Hi,
> >
> > This is a question regarding the NVT MD simulation of a polar liquid,
> i.e. a
> > molecular fluid with a dipole moment (e.g. water, nitromethane, etc.)
> >
> > When dealing with a polar slab, there are dipole corrections to
> consider.
> > This comes from a net dipole moment. Is there a similar issue with the
> NVT
> > MD simulation of a polar liquid? My co-works here (Army Research Lab)
> have
> > told me that there are such issues with there classical (non-DFT)
> simulation
> > but that the corrections become smaller as one increases the system
> size.
> >
> > Does anyone no of such discussion at the DFT level which may be found in
> > the literature?
> >
> > Bests,
> >
>
> --
> ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
> Paul Tangney
> Theory of Nanostructured Materials Facility
> The Molecular Foundry
> Lawrence Berkeley National Lab.         E-mail: PTTangney at lbl.gov
> 1 Cyclotron Road, Bldg 67                  Phone: (510) 495-2769
> Berkeley, CA 94720
> ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
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-- 
Nichols A. Romero, Ph.D.
1613 Denise Dr. Apt. D
Forest Hill, MD 21050
443-567-8328 (C)
410-306-0709 (O)
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