[CPMD-list] from younes ansari

[Axel Kohlmeyer]

On Wed, 26 Jan 2005, Younes Ansari wrote:

YA> Dear sir :

dear younes ansari,
 
YA>  I have checked out deferent methods and just got the same results. 
YA> firstly I think the definitions of atoms which is in PPLIBNEW directory 
YA> are deferent from other programs like Gaussian so I want to get the same 
YA> result in both Gaussian and CPMD.

this is simply not possible, mainly for three reasons:

- you are using pseudopotentials to describe atoms 
  with CPMD (or any other pseudopotential/plane wave/DFT code), 
  so you will only get the energy of the (valence) electrons not
  included in the pseudocore.

- you are always calculating the energy for the whole supercell
  on a grid. so the size of the supercell and the size of the
  grid (usually implicitely defined via the plane wave cutoff)
  have an influence on the total energy.

- you have different types of basis functions 
  (gaussians vs. plane waves) with different properties that
  represent parts of the wavefunction differently well.

this all, however, should have little impact on _relative_
energy differences, e.g. between different conformations
or configurations with the same number of atoms electrons 
and the same cell/pseudopotential/cutoff settings and 
the equivalent gaussian calculation. provided you are
reasonably converged with respect to the basis set. 

since the dominant application of the CPMD code are
Car-Parrinello MD calculations people care mainly about
a proper representation of the interatomic forces in
the most efficient way (so you don't need insane amounts
of computer power for your simulations). this works (usually)
very well with the plane-wave-pseudopotential approach.

please check out some of the introductory material on electron
structure calculations with plane-waves and pseudopotentials.
there are nice collections of them or links to it on
juerg hutter's homepage: 
http://www.pci.unizh.ch/gruppe.hutter/e/information.html
and on cpmd.org: http://www.cpmd.org/cpmd_thecode.html

the bottom line is, that the approach used by gaussian and
by CPMD are quite complementary, i.e. Gaussian is very 
efficient for small isolated molecules, while CPMD is very
at describing bulk liquids and solids/surfaces.

YA> 
YA>  As a result I have checked the ionization energies for Na...NA+ , 
YA> K...K+ , Rb...Rb+ and figured out that the results for K and Rb are 
YA> comparable with literatures but Na has a lot differences.

alkali metals are especially tricky with pseudopotentials. for accurate
results you usually need a semi-core pseudopotential (i.e. a 
pseudopotential, that not only includes the outer s electron but 
also electrons from the filled core shells in its valence) and/or
non-linear core correction.
 
YA> Please tell me how I can get a Gaussian type calculation (I mean the 
YA> Etot results) in the CPMD program.

you cannot. see above.

YA> secondly, I have two kinds of Au,Rb,K,....in GEODECKER for example 
YA> (K-q1,Kq9) which are the definitions are better to use for a simple 
YA> GEOMETRY calculation.

the difference between K-q1 and K-q9 is exactly the 
difference between a regular and a semi-core pseudopotential.
please have a look at the CPMD output and compare the
number of electrons used in the calculation. the q1 potential
only includes the 4s electron, while the q9 potential also includes
the 3s and 3p electrons.

which one to prefer depends on your system and the properties
you want to calculate. with the q9 pseudopotential you will
need a much larger basis set, i.e. a higher cutoff for converged
results, but then again, the pseudopotential may be much more
transferable.

for example in the case of potassium, the K-q1 pseudopotential
should good for metallic potassium but for K+ ions, you will
probably need the K-q9 since in the case of the K-q1 there
would be next to no electron density left at the K ion
which should result in a poor description of the total
valence structure.


regards,
	axel kohlmeyer.


YA>  Thanks...
YA> >
YA> 
YA> 
YA> 		
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-- 

=======================================================================
Dr. Axel Kohlmeyer   e-mail: axel.kohlmeyer at theochem.ruhr-uni-bochum.de
Lehrstuhl fuer Theoretische Chemie          Phone: ++49 (0)234/32-26673
Ruhr-Universitaet Bochum - NC 03/53         Fax:   ++49 (0)234/32-14045
D-44780 Bochum  http://www.theochem.ruhr-uni-bochum.de/~axel.kohlmeyer/
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.