[Pw_forum] Pw_forum Digest, Vol 36, Issue 38

[Stefano Baroni]

Dear Thaneshwor,

The total energy of graphite, as provided by any simple XC functional, is crap 'cause simple XC functionals fail to provide an (even approximate) account of the weak dispersion forces responsible for the inter-plane binding.

This being said, the calculation of atomic energies also poses some problems, because isolated atoms 9such as C, have open electron shells, whose multiplet structure is not straightforward to describe using DFT.

All this considered, a sensible strategy would be to compare the energy of closed-shell structures, not dramatically affected by dispersion forces, nor generacy effects. You may want, for instance, to compare the energy of a single graphene sheet (plus the appropriate amount of hydrogen molecules) with the energy of the (appropriate amount of) methane molecules (or something of that sort).

Hope you see the main point(s) and you can provide by yourself the missing pieces of the argument.

SB

PS: please, take the time of indicating the actual subject of your post in the "subject field" and remove all the unnecessary attachments from it.


On Jun 14, 2010, at 7:27 PM, Thaneshwor Kaloni wrote:

> Dear All,
> I have total energy of graphite, I need to calculate the cohesive energy of the system, for that I need to calculate the energy of single carbon atom, so would  any one help me?
> Thaneshwor Kaloni
> PhD student
> SNBNCBS
> kolkata
> www.bose.res.in 

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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