Re: [SIESTA-L] Question about LDA and GGA pseudopotential

[Dr. Y. Lin]

On Thu, 1 Feb 2007, Oleksandr Voznyy wrote:


1) Why is GGA pseudo-potential gives worse result than LDA?

What do you mean worse?
A well converged calculation (i.e. converged in E cutoff, k-grid, and 
converged vs pseudo cutoff radii as well)
LDA SHOULD give bonds (or lattice constants) 1% smaller than expt (while 
binding energies are strongly overestimated)
while GGA usually overestimates bonds by about 2% (while binding energies 
(for molecules) are much closer to expt).


First, thanks for the info. Let me explain why I think GGA gives worse 
result.


Let's take CH4 as an example. Here's the fdf input file I have
- Start of the file ---
SystemName  Methane Broyden Optimization
SystemLabel general
NumberOfAtoms   5
NumberOfSpecies 2

MeshCutoff  50 Ry

%block ChemicalSpeciesLabel
 1  6  C  # Species index, atomic number, species label
 2  1  H
%endblock ChemicalSpeciesLabel

#PAO.BasisSize  SZ
%blockPAO.BasisSizes
 C  DZP
 H   DZP
%endblock PAO.BasisSizes

AtomicCoordinatesFormat  Ang
%block AtomicCoordinatesAndAtomicSpecies
 1.0582  0.9353  0.8103 1
 1.4145  1.5662  0. 2
 1.2065  1.4452  1.7588 2
 0.  0.7294  0.6710 2
 1.6121  0.  0.8114 2
%endblock AtomicCoordinatesAndAtomicSpecies

XC.functional GGA
XC.authors PBE
PAO.EnergyShift 0.02 Ry

DM.Number.Pulay 3

WriteForces T

MD.TypeOfRun Broyden
MD.NumCGSteps 400
MD.Broyden.History.Steps 6
MD.Broyden.Initial.Inverse.Jacobian 1.0
--- End of the file ---

This input file is used along with GGA pseudo potential of Carbon.
For LDA pseudo potential case, I just change the line with Carbon
at block PAO.BasisSizes to

CDZ

Basically, this setting reduces the total basis of Carbon atom from 13 to 
8.


Then I ran siesta to relax the CH4 and measure C-H bond length after 
relaxation:


Exp.   LDACarbon   GGACarbonCCCBDB calculated
 PBEPEB/6-331G**
1.094  1.097   1.10  1.0977

It seems LDA pseudo potential did give a shorter bond-length while still 
longer than experimental value. GGA pseudo potential gives an even longer 
bond-length, which is obviously, a worse result.




2) Is it safe to use LDA pseudo-potential while setting GGA-PBE as 
x-correlation in input.fdf?

No, it is not recommended.
However, you can use the cutoff radii from LDA pseudo to generate the new GGA 
pseudo. Have you tried to compare 2 input files that you have?




I guess it is not very good to use LDA pseudo-potential while setting 
GGA-PBE as x-correlation. Thanks for confirming this. However, I'm not 
quite sure I understand your suggestion.


Let me clarify this: I'm not trying to generate pseudo-potential myself. 
So, all the input I said are for Molecular Dynamics or Relaxation. Under 
this situation, I'd say, basically, my input files are the same except for 
the basis sets. For GGApseudo, I set DZP for Carbon; and for LDApseudo, I 
set DZ.


Thanks for more info.

--


You Lin

Department of Physics
University of South Florida
4202 East Fowler Avenue
Tampa, FL 33620


Tel: (813)396-9220 [Office]

Homepage: http://shell.cas.usf.edu/~ylin

Re: [SIESTA-L] Question about LDA and GGA pseudopotential

[Oleksandr Voznyy]

1) Why is GGA pseudo-potential gives worse result than LDA?

What do you mean worse?
A well converged calculation (i.e. converged in E cutoff, k-grid, and 
converged vs pseudo cutoff radii as well)
LDA SHOULD give bonds (or lattice constants) 1% smaller than expt (while 
binding energies are strongly overestimated)
while GGA usually overestimates bonds by about 2% (while binding 
energies (for molecules) are much closer to expt).


2) Is it safe to use LDA pseudo-potential while setting GGA-PBE as 
x-correlation in input.fdf?

No, it is not recommended.
However, you can use the cutoff radii from LDA pseudo to generate the 
new GGA pseudo. Have you tried to compare 2 input files that you have?

Re: [SIESTA-L] atom Program and Pseudopotential error

[Edan Scriven]

You have to be careful with whitespace in the input file. The following
works for me:

 pg Li TM2 Pseudopotencial GS ref
 tm2
 Li ca
 0.0
 1 2
 2 0 1.00
 2 1 0.00
 1.54 1.54 0.00 0.00
12345678901234567890123456789012345678901234567890 Ruler

Also, as Bipul said, you'll probably get errors in an actual SIESTA
calculation unless you add some more angular momenta:

 1 4
 2 0 1.00
 2 1 0.00
 3 2 0.00
 4 3 0.00

Alternatively, you should be able to avoid that by adding a PS.lmax
block to your .fdf file (the SIESTA user guide shows you how).


Edan.


Saswata Bhattacharya wrote:
dear friends, 
i have got a typical problem for the last few months.whenever i go to
run atom program for Li i use the following input given below---
  
 pg Li TM2 Pseudopotencial GS ref
 tm2
 Li ca
 0.0 0.0 0.0 0.0
 1 2
 2 0 1.00
 2 1 0.00
  1.54 1.54  0 0 
12345678901234567890123456789012345678901234567890 Ruler
  
but each time i get the same output message given below---
cp: cannot stat `VPSOUT': No such file or directory
cp: cannot stat `VPSFMT': No such file or directory
  
i cant understand what is just happend and i shall be highly grateful
if anybody tells me about the actual mistake i have done..
with regards,
Saswata
  
   
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Answers