[Pw_forum] discrepancy in molecular binding energy between pwscf and localized basis set program

[Garold Murdachaew]

Dear Stefano, Axel, and Agostino,

I should have mentioned that the molpro calculation is done with a large
basis set (I checked for convergence), and, as always necessary for these
types of calculations, the counterpoise correction for BSSE.  The fact 
that the LDA calculations (where the same basis set and CP correction was 
also used in molpro) agree show that the disagreement in the PBE values 
between the programs is not due to a basis set problem in the molpro 
calculation.

By the way, I realize that DFT cannot describe this van der Waals
interaction: the correct result from molpro at CCSD(T) level is -21 meV
while a more accurate ab initio calculation of Slavicek et al. gives -22 
meV and the very accurate Aziz empirical potential fitted to rovibrational
spectra of Xe dimer gives -24 meV; note that all of these numbers are
without ZPE.  However, since I am trying to assemble a hybrid method which
relies on some calculations from pwscf (for Xe/Cu(111) physisorption),
supplemented with the neglected van der Waals corrections from molpro, it 
is important to be able to reproduce the same DFT numbers in both 
programs.  This is especially necessary since pwscf with PBE gives -40 
meV/atom and -34 meV/atom for the V3xV3 Xe/Cu(111) physisorption energy 
for top and fcc sites respectively.  Thus, differences of a few meV become 
important in this problem.  I am hoping that all the van der Waals-type 
corrections will bring the PBE top-site value down to something close to 
the experimentally measured value of about -183 meV/atom, as well shifting 
the minimum of the binding energy curve to something close to that 
measured experimentally.

Thanks to all of you for your comments.  The question still remains open, 
however...

Best regards,
Garold



On Tue, 12 Jun 2007, smigliore at unimo.it wrote:

> Dear Garold,
>
> let me say that I am in complete agreement with what suggested by Axel. BSSE can
> also account for a so large reduction in the binding energy; see, e.g., Chem.
> Phys. 324, 455 (2006).
>
> Best Regards,
> Agostino Migliore
> PS: There are also many PWscf users concerned with energies on the meV scale.
>
> Quoting Garold Murdachaew <murda at sissa.it>:
>
>> Dear pwscf users,
>>
>> I am doing some calculations for relatively weakly interacting species and
>> decided to compare to what I obtained with localized wavefunction based
>> programs such as molpro.  For example, I did calculations of the binding
>> energy of Xenon dimer (at the minimum separation of 4.42 Angstroms).
>> I obtained (in meV):
>>
>>  	molpro	pwscf3.2
>> LDA	-34	-34
>> PBE	 -4	-11
>>
>> As can be seen, the LDA calculations agree perfectly but the PBE pwscf
>> calculation gives an interaction energy almost three times more negative.
>> (I realize that for most pwscf users who are usually only concerned about
>> energies on the eV rather than on the meV scale these differences may seem
>> insignificant.)
>>
>> Here is my PBE pwscf input file for the Xe dimer (the monomer and the LDA
>> inputs are similar):
>>
>> &CONTROL
>>    calculation   = 'scf',
>>    prefix        = '$HEAD',
>>    title         = '$HEAD',
>>    restart_mode  = 'from_scratch',
>>    outdir        = '$TMPDIR/',
>>    PSEUDO_DIR    = '$PSEUDODIR/',
>> /
>> &SYSTEM
>>      ibrav     = 0,
>>      celldm(1) = 25.d0,
>>      nat       = 2,
>>      ntyp      = 1,
>>      ecutwfc   = 30.0d0,
>>      ecutrho   = 300.0d0,
>>      occupations = 'smearing',
>>      degauss     = 0.001D0,
>>      smearing    = 'mp',
>>      nbnd        = 12,
>> /
>> &ELECTRONS
>>      conv_thr    = 1.0d-8,
>>      mixing_mode = 'plain',
>>      mixing_beta = 0.3d0,
>> /
>> ATOMIC_SPECIES
>> Xe 131.30   Xet2tPBE.RRKJ3
>> ATOMIC_POSITIONS { angstrom }
>> Xe     .000000000    .000000000    .000000000  0  0  0
>> Xe     .000000000    .000000000   4.420000000  0  0  0
>> K_POINTS { gamma }
>> CELL_PARAMETERS { cubic }
>>     1.00000    0.00000    0.00000
>>     0.00000    1.00000    0.00000
>>     0.00000    0.00000    1.00000
>>
>> As can be seen, the molecule is in a cubic box of edge length 25 bohr and
>> thus there is sufficient vacuum to minimize the spurious interactions with
>> its periodic images.  I played with the convergence setting conv_thr but
>> setting it to a factor of 100 tighter had very small effect.  Changing the
>> degauss setting had no effect on energy (only Fermi level was changed.)
>> However, when I increased  ecutwfc and ecutrho to 70 and 700 respectively,
>> I obtained a slight change (-10 meV rather than -11 meV).  Can somebody
>> please tell me how I can reproduce my PBE  localized basis sets
>> calculation.  Do I need to make some changes to the pseudos?  Or is this
>> what I can expect from gradient-corrected calculations (such as PBE) due
>> to some sort of errors in estimating the gradients (which are not needed
>> in the LDA calculation)?
>>
>> Thank you for you assistance.
>>
>> Best regards,
>> Garold Murdachaew
>>
>> ps:  Here are the details of the pseudo I used in pwscf:
>>
>> LDA pseudo: XeLDA.RRKJ3.UPF: nonrel, nc, nlcc=t, valence=8, maxl=2
>> PBE pseudo: Xet2tPBE.RRKJ3 : nonrel, nc, nlcc=t, valence=8, maxl=2
>>
>> --
>> Garold Murdachaew
>> Postdoctoral Fellow - Condensed Matter Sector
>> SISSA-ISAS (International School for Advanced Studies)
>> New Building Room 208
>> Via Beirut 2-4, 34014 Trieste, Italy
>>
>> tel   : +39 040 3787 477
>> fax   : +39 040 3787 528
>> email : murda at sissa.it
>>
>>
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>
>
>
>
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-- 
Garold Murdachaew
Postdoctoral Fellow - Condensed Matter Sector
SISSA-ISAS (International School for Advanced Studies)
New Building Room 208
Via Beirut 2-4, 34014 Trieste, Italy

tel   : +39 040 3787 477
fax   : +39 040 3787 528
email : murda at sissa.it