[gpaw-users] Potential energy comparation

[Marcin Dulak]

Hi,

the short answer will be: NWchem calculates and prints energy of all 
electrons in the system,
GPAW print only a relative energy (in the output as Zero Kelvin:) with 
respect to some reference energy
(in the output as Reference Energy:) based on constituent atoms, 
relaxing only valence states:
https://wiki.fysik.dtu.dk/gpaw/features_and_algorithms.html#introduction

If you add Zero Kelvin + Reference Energy you will get closer to NWchem 
total energy but these are not the same things.
One can never compare total energies from different programs
(except maybe for purely analytical methods like gaussian basis set 
implementation of Hartree-Fock in a given basis set).

Please follow this tutorial first: 
https://wiki.fysik.dtu.dk/gpaw/tutorials/atomization/atomization.html
Then, as a good exercise consider calculating atomization energies of 
small molecules (https://wiki.fysik.dtu.dk/gpaw/setups/molecule_tests.html).
You can do that on the command line (see for example atomization energy 
of CO2
https://listserv.fysik.dtu.dk/pipermail/gpaw-users/2012-May/001410.html
and a general introduction here 
https://wiki.fysik.dtu.dk/ase/ase/cmdline.html).
Note several important features: broken cell symmetry (needed for some 
atoms), width=0.0, rather fine grid spacing h.

You can start building your explicit python scripts based on this 
template for example:
https://listserv.fysik.dtu.dk/pipermail/gpaw-users/attachments/20111214/ab177be4/attachment.py
or
https://listserv.fysik.dtu.dk/pipermail/gpaw-users/attachments/20110926/c8fc2556/attachment.py

Best regards,

Marcin

On 07/17/12 18:37, davi.fisica at gmail.com wrote:
>
> Hi folks, i'm new user and had one basic question: why potential 
> energy calculated with gpaw is so different of energy calculated for 
> others softwares?
>
> Using Nchem and ASE i found 5036 eV for CO2 molecule. With GPAW i 
> found 24 eV, what's up?
>


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Marcin Dulak
Technical University of Denmark
Department of Physics
Building 307, Room 229
DK-2800 Kongens Lyngby
Denmark
Tel.: (+45) 4525 3157
Fax.: (+45) 4593 2399
email: Marcin.Dulak at fysik.dtu.dk

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