[Pw_forum] some naive problems

[Stefano de Gironcoli]

Dear Lihui Ou and Osman Baris Malcioglu,
    let me re-state Osman's reply in a way I like more...
    Plane-wave codes (and for that matters also LMTO, FLAPW codes  etc) 
are built so as to  describe periodic systems and CAN deal with isolated 
molecules by the use of supercells large enough that the periodic images 
do  not interact significantly. There are other codes (I guess NWChem is 
one of them) that work for isolated system only and CANNOT describe 
periodic systems. This may be a problem when wishing to compare isolated 
and periodic systems on the same footing...
   A plane-wave basis-set is an expensive one but is unbiased (describe 
with the same accuracy every point in space) and its accuracy is 
extremely easy to control (just increase your cutoff energy until the 
property you are interested in converges). A lot of effort in the past 
has been put in the development of pseuodopotential theory that allows 
to treat any element  in the periodic table with good accuracy (some 
already using norm conserving pseudos while some others may need 
ultrasoft ones). Pseudopotential method is not an all-electron method  
but very often the results are very close to AE and always, to my 
experience, physically meaningful. An evolution of the US 
pseudopotential method is the PAW formalism that can be considered an 
all-electron method. We are currently implementing this formalism in PWscf.
    Localized basis (such as gaussian or slater-type-orbitals) are more 
compact but are more difficult to control and require a lot of know-how 
and experience (double-z ? triple-z ?  triple-z+polarization ?) in order 
to avoid basis-set superposition error and the like.

   Parameters controlling plane-wave calculations:
   1) kinetic energy cutoff:  as said it defines the dimension of the 
basis set and you just need to increase it until you are satisfied. The 
required cutoff is a PROPERTY OF THE PSEUDOPOTENTIAL USED not of the 
particular system under study. It stems from the need of describing 
accurately the pseudopotential wavefunctions.
   2) k-point sampling : it's a property of your system (insulator, 
metal, isolated, 2-d, 1-d) not particularly of the pseudopotential 
used.  For isolated molecules (that is if the supercell is large enough 
and hence the IBZ small enough) Gamma-only sampling (for which special 
tricks to speed up the calculation can be used) is a good choice. If it 
is not the case, this means that periodic images  are interacting... 
then increase the cell dimension and stick to Gamma sampling.
  As for Hybrid functionals: currently only NC pseudopotentials  are  
implemented, sooner or later US (or most probably directly PAW) 
implementation will follow... any skilled volunteer
would be welcome.
  hope this helps,
    stefano de Gironcoli,  SISSA and DEMOCRITOS

mbaris at metu.edu.tr wrote:
> 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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