[CPMD-list] How to find the ground state energy of a single atom

[Axel Kohlmeyer]

On Fri, 2 Mar 2007, Sirilak Sriburadej wrote:

SS> 
SS> Hello, CPMD users,
SS> I am very new to the CPMD program. Moreover, I have very little knowledge about how to use
SS> the program. If my question was too easy, I would like to apologies for it. However, it would

so i suggest you start with the tutorial/introductional material 
available on the web to understand what is going on.

SS> be grateful if you could answer my questions. What I would like to know if I want to
SS> calculate the ground state energy(total energy) of a single atom such as H, He, Li, ..,O, F,


the method implemented in CPMD (DFT with a plane wave basis set 
and pseudopotentials) does not lend itself well for calculation 
of (absolute) total energies of isolated species.

SS> .
SS> 
SS> The Input file of mine is

[...]

your input looks reasonable. you should note that
at 70 ry plane wave cutoff, the total energy is not
yet well converged. in fact, it will take a much much
larger basis set to converge the energy. however, for
the typical applications of CPMD (md and geometry optimization)
this does not matter so much since the forces converge
much faster.

SS> My output is
SS> 
SS> (K+E1+L+N+X)           TOTAL ENERGY =          <?xml:namespace prefix = st1 ns =
SS> "urn:schemas-microsoft-com:office:smarttags" />-24.80040831 A.U.

(as a side note, it would be less annoying for people that read
mail with a text based mail program (like me and a few others here),
if you'd set up your mail program to emit text and no html/xml).

[...]
SS> 
SS> The result from calculation was -24.80040831 a.u.. As the result, the value from calculation
SS> and from the experiment (-75.067 a.u.) was unacceptable difference. I had tried find out the

what experiment??
as mentioned before, you are doing a pseudopotential calculation, so 
the value are seeing, does _not_ include the contribution of the core
electrons.  

SS> value from various atom. I used the same style of the input files : the different was only
SS> the pseudopotentials file and LMax. I want to know
SS> 
SS>  1. Are there any others essential parameters in order to figure out the ground state
SS>     energy(total energy) of a single oxygen(O) atom?
SS>  2. Regarding the calculation of ground state energy of a single atom and others atom, do we
SS>     used the same procedure as the calculation of the ground state energy of a single
SS>     oxygen(O) atom? Are there any other essential characteristic of atom that we have to take
SS>     into account? If so, what are they?

the total energy value for single atoms mostly depends on how the 
pseudopotentials were created, but also a few other parameters.
if total energies are your ultimate goal, than you are using the 
wrong program. please note, that total energy _differences_ are
usually quite accurate, provided all technical parameters stay the
same, i.e you compare different conformations or atomic positions.

SS> I would really appreciate if you could give me pieces of advice as well as I am absolutely
SS> delighted to listen to every suggestion.

cheers,
   axel.

SS> Best regards,
SS> Biwei  Huang


p.s.: i am curious. how come, your name here does not match neither
the one in the from name, nor your email account? 
...or is there something, i don't get?

SS> 
SS> 
SS> ____________________________________________________________________________________________________
SS> The average US Credit Score is 675. The cost to see yours: $0 by Experian.
SS> 

-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.