Re: [ESPResSo-devel] User Guide on ELC and noneutralization

[Vincent Ballenegger]

Hi Stefan,

> Also, there is this thing with the "-noneutralization" command when dealing
> with non-neutral systems. There is no information about that command in
> the user guide as well. I now figured out for myself, that one has to add
> that command, but I find its name somehow misleading. Actually when
> activating that feature there IS a neutralization done by the artificially
> added homogeneous background, so I would say its name should rather be
> neutralization, or even better let it get that information from the total
> charge of the system itself.

The name "-noneutralization" for this option is indeed somewhat misleading. It 
originates from the fact that this option kills an unwanted effect (creation 
of a parabolic potential) associated to the homogeneous neutralization  
inherited from the underlying P3M method in P3M/ELC. 

The way one can define energies and forces in a simulation of a slab system 
carrying a net charge is explained in details in paper

V. Ballenegger, A. Arnold, J.J. Cerdà, "Simulations of non-neutral slab 
systems with long-range electrostatic interactions in 2D periodic boundary 
conditions", J. Chem. Phys. 131: 094107 (2009)

The option -noneutralization corresponds to use what is called the "background 
charge correction" (bcc) defined by eq. (28) in that paper.

Personally, I think that this option should be activated automatically in 
Espresso when simulating a charged slab with P3M/ELC. The name 
"noneutralization" could be dropped entirely. When simulating a slab system 
with a net charge, Espresso could output a message "Warning: system with a 
non-zero net charge. Using background charge correction, see Ref. ...".

Regards,
 Vincent Ballenegger