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Hi,

I have two questions:

1) I am told that for ionic relaxation LDA is better than GGA.
Is that so?

2) How do I switch from GGA to LDA?
I usually use the potentials that are in the pot_PBE/ directory.
I assume that this induces GGA, right? Should I instead use the
potentials in pot/ directory for doing LDA?


Thanks,
Rob.

You are using PBE/PE potential. PBE is a special case(simplified) of PW91. The possible GGA tags are: PW/PB/LM/91/PE/RP. That means you can use say RP functional by defining GGA=RP in INCAR. GGA potential are LDA + GGA corrections. The LDA part is parametrized by Perdew Zunger method, not Perdew's pade approximation (mentioned in
http://cms.mpi.univie.ac.at/vasp-worksh ... tabase.pdf ). You can switch to LDA by keeping GGA= (blank) in INCAR as well as in POTCAR and in POTCAR LEXCH=CA; these potential dataset are provided in different directory. for paw PPs, there are 3 XC types available by default, LDA, PW91 and
PBE. only RP has to be specified via INCAR (GGA=RP).
You can switch between ULTRA/PAW by LPAW/LULTRA in POTCAR.

The general statistics about the comparison of performance of LDA(LSDA)/GGA(GGS):

(1) Cohesive Energy:
GGA: 4% larger (~0.3 eV)
LDA: 15% larger.

(2) Structure:
GGA: 1% larger
LDA: 1% shorter

(3) Energy Barrier:
GGA: 30% low
LDA: 100% low

(4) Ex
LDA: 5% larger

(5) Ec
LDA: 100% larger

The manifestations (LDA) are: it favors close-packed structure. Fe LDA is fcc (experimentally bcc ferromagnetic)
energy barriers too low (no chemical accuracy)
negative ions are unstable

Bandy wrote:
I have done this:

$ grep GGA POTCAR INCAR
POTCAR: GGA =
INCAR: GGA =
$ grep LEXCH POTCAR INCAR
POTCAR: LEXCH = CA

However, I then get this warning (error?) message:

I find this quite scary, because I don't know exactly what I'm doing.
What is this warning about? Is it serious problem?
Or does one have to take these warnings for granted when
switching to LDA?

Rob.

Did you use LDA in specific directory in POTENTIAL database?

Bandy wrote: I have no idea what you are asking in this sentence.

Maybe I'm too innocent, but could you please rephrase it to make
it easier for me to understand.

By the way: I do not generate my own POTCAR files; I use the ones
provided by VASP (usually the ones from pot_PBE/ directory).

Thanks,
Rob.

You should have LDA PP in a separate directory (PAW LDA) when you have downloaded PPs from vasp website
ftp://cms.mpi.univie.ac.at. I have PPs in 5 different directories:
paw_pbe, pot_GGA, pot_LDA, pawGGA, pawLDA

paw_pbe, pawGGA, pawLDA are PAW PP
pot_LDA, pot_GGA ultrasoft PP

Hi Rob,

Let me tell you in detail. In fact the possible LDA tags (LEXCH) are: Slater-XC, HL (Hedin Lundquist [1971]), CA (Ceperly Alder parametrized by Perdew Zunger ), WI (Wigner perturbation), PB (Perdew Becke), PW (Perdew-Wang 86), LM (Langreth-Mehl-Hu), 91(Perdew-Wang 91). Similarly possible GGA tags are:
PW (Perdew-Wang 86), PB, LM, 91, PE, RP.
Also you can define LEXCH by numbers like LEXCH=0(local format), 1(CA; default), 2, 3, 4(HL Hedin Lundquist), 5, 6(Wigner), 7 (Pade approximation of Perdew) etc. similarly LEXCHV=0, 1, 2, 3, 8,9

Hope somebody who has tried these PPs can expalin well their usage as I am restricted to 1-2 PPs only.

I would strongly suggest to choose the most appropriate pseudopotential type from the data base instead of setting the GGA flag in INCAR. Please have a look at the pseudoppdatabase.pdf file which you can download from http://cms.mpi.univie.ac.at/vasp-workshop in order to check which PP is most appropriate for the calculations you want to perform.

Hi,
I have made several calculations with POTCAR PAW LDA. In what way the results of my calculations would be different if i had used POTCAR PAW GGA? Is the lattice optimization of a structure calculated with PAW LDA different of the SAME strucrure calculated with PAW GGA?
Thanks