[Pw_forum] MD simulations of polar liquids

[Paul Tangney]

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,
>

-- 
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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
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