[Pw_forum] some naive problems
mbaris at metu.edu.tr
mbaris at metu.edu.tr
Fri Nov 2 09:17:31 CET 2007
Dear Lihui Ou,
> Do pwscf only periodic hybrid density functional theory calculation,
> it could do any other general density functional theory calculation?
I am not sure if I got the point of your question correctly, but Plane
Wave SCF program can only solve systems thorough plane wave
approximation, i.e. not through an all-electron calculation, and for
simplification purposes (that is, in order to have tangible Fourier
transforms etc.) periodicity/symmetries are employed. This is more than
acceptable for crystals, but for macromolecules etc, you should be
extra careful setting the boundaries (the most simple limit is this: a
very big box surrounding the molecule you are interested in will
somewhat approximate a standalone molecule, however the size of the box
is limited by a combination of how the plane wave approximation is
employed in pwscf and bare computational limits, you need to find the
optimum value). There are other packages available such as NWChem or
Gaussian for all-electron calculations. Pseudo potential wise, you can
create a pseudo potential to your desire, as long as you can validify
your results. Please see the documentation.
> Is there any principle about setting of k-point and cut-off energy?
Well, yes. The idea is: You should minimize the number of k-points and
cut-off (and a number of other parameters depending on the system you
are studying) in order to save computer time whilst maintaining
physically meaningful results. The "physically meaningful result" part
depends on your problem, keep in mind that total energy in DFT is not
very well defined, due to nature of Kohn-Sham orbitals, only the
differences in energy are trustworthy, or the observables, such as unit
cell volume. Generally you are advised to do a number of calculations
with varied parameters in order to obtain a converged set before
starting the problem itself (i.e. if you are using lattice parameter as
your observable, find the lattice parameter that minimizes the energy
for each ecutwfc-k-point combination. You will see that after a certain
limit, your lattice parameter will not change much, that value is the
minimum you can use, you may later need to increase it, depending on
your problem).
> Best wishes
> Lihui Ou
Hope this was helpful,
Best,
Osman Baris Malcioglu
Ph.D. Candidate
METU, Physics
Ankara, Turkey
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