[Pw_forum] Ewald and Coulomb

[Axel Kohlmeyer]

2012/1/15 jiayudai <daijiayu at nudt.edu.cn>:
>
> Dear Axel and Yun-Peng,
>
> Thanks for your physial translation. Now, i think i can understand the
> physical figures in this problem. As axel said, the forces can be correct,
> but the energies are wrong. However, what i want to get is the pressure from
> positive ions. Thus, i should need the correct energies at first. For the
> treatment of Coulomb interations, we usually do the summation within a
> cutoff of a distance if we only use the formation of 1/r. This is an
> approach but it can solve the problem of divergence. Therefore, i think we

this is not *solving* the problem, it is *ignoring* it.
and much worse. since coulomb interactions are
so very long-range, a cutoff will induce *massive*
fluctuations of properties like pressure relative
to the choice of cutoff (and what kind of cutoff,
spherical? or use the minimum image cell?)
and that already for a neutral system.

i suggest you set up a classical simple test
case with a simle rocksalt like crystal structure
and check out how much the pressure depends
on the choice of cutoff with and without ewald
summation. i am certain, this will change your
mind and you will be shocked.

axel.



> can change the ewald formula in DFT using 1/r (with a cutoff). Is it?
>

-- 
Dr. Axel Kohlmeyer
akohlmey at gmail.com  http://goo.gl/1wk0

College of Science and Technology
Temple University, Philadelphia PA, USA.