[Pw_forum] comparing gaussian03 with pwscf

[Axel Kohlmeyer]

On Mon, 5 May 2008, O. Baris Malcioglu wrote:

BM> Hi,
BM> 
BM> Maybe some details on how you are calculating the system in Gaussian 
BM> will be helpful, especially the basis set and the X-C functionals. I 
BM> suspect the problem arises from the intrinsic difficulty of describing 
BM> Carbon atoms and Lanthanium at the same time using all-electron 
BM> wavefunctions. You should pay extra attention how your carbon cores are 
BM> described in a set that is tailored for Lanthanoids, or vice versa.

baris,

i beg to differ. there may be basis set completenes issues when 
using a too small basis set, but why should you need to tailor
the carbon basis set for lanthanoids? the whole point of having
defined basis sets is, that those _are_ balanced. so that should
at most be a minor effect.

i would rather expect the differences arising from much 
more obvious issues. i.e.,

a) the fact that you have to project your plane wave wavefunction
   on an atomic basis set and that this projection cannot be
   complete, since you don't have a complete basis set that
   you project on. rather you use a minimal basis built from the
   atomic pseudowavefunctions. you can easily lose 1-2 electrons here.

b) as paolo already noted, the differences between different ways
   to perform the population analysis. e.g. mulliken vs. loewdin vs. 
   nbo vs. ...

if you have access to the CPMD (www.cpmd.org) code you can easily see 
how large those effects can be, as the code allows to project on
either atomic pseudo wavefunction or a minimal slater basis (or any
gaussian basis you define in the input) as well as it supports mulliken
and loewdin like analysis. comparing the dependence of the thusly 
calculated partial charges from plane waves on the basis set and 
method is quite educational... (not to mention the 
impact of pseudopotentials, plane wave cutoff etc...).

we are frequently tempted to put too much trust into the many
digits of numbers that we calculate. their absolute accuracy
can be _much_ less. 

cheers,
   axel.


BM> 
BM> O. Baris Malcioglu
BM> SISSA CM sector,
BM> Trieste,  Italy
BM> Tel:+390403787374
BM> 
BM> > On May 3, 2008, at 1:25 , JAIME GUSTAVO RODRIGUEZ ZAVALA wrote:
BM> >
BM> >   
BM> >> When analyzing the endohedral La at C60 molecule I
BM> >> obtain, using the gaussian03, a charge transfer from
BM> >> the La atom to the C60 cage, leaving the La atom with
BM> >> a charge of +2.12. However the pwscf charge
BM> >> calculation (with the La.pw91-nsp-van.UPF) gives also
BM> >> a charge transfer from the La atom to the C60 cage,
BM> >> but leaves the La atom with a charge of +1.08. Is this
BM> >> a natural difference between gaussian03 (natural
BM> >> population analysis) and pwscf (Lowdin analysis)?
BM> >>     
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BM> 

-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
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