[ase-users] [ase-developers] ASE calculator interface proposal

[John Kitchin]

>   I agree that a calculator should be aware of when a change has occurred
> that makes the last calculated values inconsistent with the current state
> of the calculator.
>
> * Standard parameters
>
> * kpts
> 1. Should there be a k-point density parameter? Maybe kpts=3.5 as in
>    3.5 \vec k-points per Å^{-1}?
>
> I think this is a good idea. We often use kpts per reciprocal atom instead
> of per reciprocal angstrom. maybe (n1) or n1 could refer to one case, and
> (n1, 'kppra') to another case. We have a function that calculates this for
> us.
>
>
> What do you mean "kpts per reciprocal atom"?
>


this is something I picked up from ATAT (cluster expansion code). Suppose I
have a primitive fcc cell with 10x10x10 kpts. That is 1000 kpts per
reciprocal atom. If I now make a 1x1x2 super cell, the corresponding
commensurate grid is 10x10x5, which is only 500 kpts, but there are 1/2
reciprocal atoms, so I still have 1000 kpts per reciprocal atom. I have an
algorithm that tries to generate a k1xk2xk3 grid that provides at least
that number of kpts per reciprocal atom, and has an approximately even kpt
density in each reciprocal vector.

Here is my algorithm.

https://github.com/jkitchin/jasp/blob/master/jasp_kpts.py#L165

I got used to this  before I got used to a reciprocal angstrom formulation.


>
>
>  a. [(k11,k12,k13),(k21,k22,k23),...]: Explicit list in units of the
>    reciprocal lattice vectors
>
> You should also be able to specify cartesian coordinates for kpts.
>
>
> I don't see how we can make that work also with the kpts keyword.  Are you
> suggesting another keyword?  One could just do kpts=np.dot(atoms.cell,
> cartesian_kpts.T)/(2*np.pi).
>
>
I guess I am suggesting another keyword. The KPOINTS file in VASP takes
both formulations. It is true it is only some linear algebra that separates
them. you might need another .T in the expression above to get the kpts
list in rows though.
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