[Pw_forum] The question about difference between diamond and graphite

[Alexander G. Kvashnin]

Dear PWSCF community

In my study I got a problem which I can't solve by myself,
unfortunately. I hope you can give me some tips where I made a
mistake. I have to to investigate the energy difference between
graphite and diamond, but I can't get trusted result.First of all, I
used LDA PP (PZ), I tested it by subsequently increasing of energy
cutoff (from 30 Ry to 120 Ry) as well as increasing of k-points mesh
(4x4x1 for graphite, 4x4x4 for diamond; 6x6x2 for graphite, 6x6x6 for
diamond;
8x8x3 for graphite, 8x8x8 for diamond; 10x10x4 for graphite, 10x10x10 for
diamond).
I got the difference between energies of diamond and graphite and tried to
understand it
trend (please, see attached figure). As you can see from the figure
difference between energy diamond and graphite is almost zero so I
can't say with confidence what structure is more favorable.
This result different from other theoretical and experimental
studies, therefore it seems that I did something wrong...

 My result: ~ 0.001±0.001 eV/atom

 _______________________________________________

| Energy difference (eV/atom) | Method      |  Reference     |
|            0.03                        |Experiment | PRB29_6996  |
|            0.01                        |  LDA-PP    | PRB29_6996  |
|            0.04                        |  LDA-PP    | PRB54_14994|
 -----------------------------------------------------------------------------------

Input files for diamond and graphite are attached to the letter.


-- 
Sincerely yours
*Alexander G. Kvashnin*
--------------------------------------------------------------------------------------------------------------------------------
First year of master degree study
Moscow Institute of Physics and Technology          http://mipt.ru/
141700, Institutsky lane 9, Dolgoprudny, Moscow Region, Russia

Junior research scientist
Technological Institute for Superhard
and Novel Carbon Materials
http://www.ntcstm.troitsk.ru/
142190, Central'naya St. 7a, Troitsk, Moscow Region, Russia
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