plane wave and band: why(how) are they different?
Gets Around
2:49:33 PM PST - Sun, Jan 29th 2012
I have a question to Mr. Bylaska. I've been trying to wrap my mind around it: why are plane wave code and band codes (sort of) different in NWChem, e.g. they implemented as different modules? I can;t quite understand that. All the codes (CRYSTAL, VASP etc.) regardless whether they use basis set or waves, always sample grid of k points in reciprocal space. I assume band in NWchem does the same, but what's the purpose of plane wave code alone then? In other words, what's the catch here, aren't they simply the same thing only plane wave (PSPW) uses one k point, whereas band (NWPW) uses the specified grid? Could Eric elaborate or give some references? What would be similarities and differences, especially in applications?

thanks

Gets Around
3:09:29 PM PST - Sun, Jan 29th 2012
I just looked at the manual: they both even go under the same
NWPW
END
input to make matters even more confusing

I f I am correct, the only differentiating thing in the input is
BRILLOUIN_ZONE 
  ZONE_NAME <string name default 'zone_default'> 

  (KVECTOR <real k1 k2 k3 no default> <real weight default (see input description)> 

   ...) 

END

so again, why use one versus the other, e.g. why not always use band?

Gets Around
12:28:15 PM PST - Mon, Jan 30th 2012
pspw and band
For the most part, everything you can do with PSPW can be done with the BAND code. The reason their are two different code is that PSPW is based on real-complex FFTs and the BAND uses complex-complex FFTs. The PSPW code is slightly faster since the wavefunctions can be describe with half the amount of space.

The codes are fairly interchangeable, so you should be able to run a sequence of pspw and band jobs, e.g.

task pspw energy
task pspw optimize
task band optimize
task pspw energy
...

w/o the code crashing.

  • Guest -
4:28:10 PM PST - Mon, Jan 30th 2012
I see, thanks Eric. Can I use hybrid functionals with band? Also, I am interested in getting out DOS plots, can I get them out for selected atoms, how would the input look like? That is, besides total DOS, which there is an example for online, can I hand pick atoms or their groups for DOS? Total DOS is rarely very informative


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