Implementation of PBE0 functional in NWCHEM

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Posts 37
2:17:28 AM PDT - Tue, Jul 23rd 2013
Hello all,


I would like to extend a work involving the PBE0 functional started with MOLPRO and to continue it with NWCHEM. However, it seems that the energies provided by the default PBE0 methods are not the same for a calculation of the ground state of the carbon atom.



Here are the parameters of the DFT functional in MOLPRO: 

Density functional group PBE0=PBEX+PBEC

Functional: PBEX(Automatically generated (new) PBEX)      Gradient terms: 1

Functional: PBEC(Automatically generated (new) PBEC)      Gradient terms: 1

Exact exchange will be calculated, factor:  0.2500

Functional: PBEX                      Factor:  0.7500

Functional: PBEC                      Factor:  1.0000


and the same in NWCHEM:

                        PBE0 Method XC Functional

                    Hartree-Fock (Exact) Exchange  0.250

         PerdewBurkeErnzerhof Exchange Functional  0.750

           Perdew 1991 LDA Correlation Functional  1.000 local

          PerdewBurkeErnz. Correlation Functional  1.000 non-local

What are the differences between the two software packages' default implementation of PBE0, i.e. can their PBE0 definitions/implementations be adjusted?

I tried the default B3LYP functional as well and I have the same problem.

My nwchem input for carbon atom is:

geometry

zmatrix

C;

end

end

basis

C library 6-31g

end

scf

thresh 1.0e-5

end

task scf

dft

xc pbe0

convergence energy 1.0e-6

convergence gradient 1.0e-6

end

task dft

Thanks in advance,
Sincerely yours,
Guillaume

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Posts 1676
9:18:49 AM PDT - Tue, Jul 23rd 2013
Guillaume
could you please try an atom where all shells are completely filled, e.g. a gas such as
helium or neon?

If you need to compute DFT atomic energies, the situation requires to somewhat enforce spherical symmetry
and there a few different ways to do it ... and it is possible that Molpro is doing that.

In the case of atoms with completely filled shells there is not any ambiguity of the sort mentioned above.

Cheers, Edo

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Posts 37
1:38:13 AM PDT - Wed, Jul 24th 2013
Deers Edo

I have already done a calculation of the ground state of a neon atom and I have the same problem.

Moreover, I compute using a cartesian basis set for HF method in MOLPRO and NWCHEM. For the HF method, the value of energies are exactly the same (in the case of carbon and neon atom).

I am going to take a look at the spherical symmetry in MOLPRO.

Thanks,
Guillaume

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Posts 1676
9:24:23 AM PDT - Wed, Jul 24th 2013
Guillaume
If the Ne energy disagrees, we can exclude the atomic symmetry as a factor.
A possible source of discrepancy is the LDA component used by the Correlation PBE functionals.
NWChem makes use of the Perdew-Wang 1991 functional for the LDA bit of PBE Correlation,
can you tell what is Molpro using?
Edo

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Posts 1676
10:52:25 AM PDT - Wed, Jul 24th 2013
Agreement to 5 digits
Guillaume,
I have tried the calculation myself on both codes and I am getting a 5 digits agreement on the total energy of
-128.78985 hartrees

NWChem output file (input echeod at the top)
http://www.nwchem-sw.org/images/Ne.nwout

Molpro input file
http://www.nwchem-sw.org/images/Ne_molproinput.nw

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Posts 10
12:53:54 PM PDT - Mon, Aug 19th 2013
Which version of MOLPRO are you using? MOLPRO 2009 and older versions define a different PBE0 functional.

http://www.molpro.net/pipermail/molpro-user/2009-December/003459.html


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