[Pw_forum] some naive problems

[mbaris at metu.edu.tr]

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