[Pw_forum] questions about lda+u calculation

[Matteo Cococcioni]

Hi Maxim,

equivalent atoms are those whose coordinates are related by symmetry 
operations. so they occupy
crystallographically equivalent positions.
In a supercell (as the one we use to construct the response matrix and 
to compute U) you may have
more than one equivalent atom. Of course they would give exactly the 
same response to the perturbation so you
only need to perturb one of them. Still to construct the matrix you need 
to be able to recognize which atom is
equivalent to which and attribute the same response.

so in few words, this is done to save computational time. which is 
always good.

Matteo

Maxim Ivanov wrote:
> >
> > if you have a look to the file resp_mat.f90 you can check the reason
> > to have atomic positions read from file.
> > there are needed to calculate the matrix elements of chi and chi0.
> >
> Nevertheless I don't understand why it is necessary to know atomic 
> position to calculate chi which is dn/dalpha derivative. Isn't it a 
> slope of n(alpha) function?
>
> >
> > if you are considering a FeO unit cell with 8 ions you can perturb
> > only one Fe site (you don`t need to look at occupancies on O sites)
> > if  all the other Fe are equivalent to this one.
> > one have to perturb once all the ions which are not equivalent.
> >
> In this case what is the definition of equivalence of the ions?
>
> Maxim
>