[Pw_forum] Energy of CO Crystal

[Axel Kohlmeyer]

On Fri, 9 Jan 2009, Amos Leffler wrote:

AL> Dear Forum,

dear amos,

AL>        I have been trying to calculate the energy of a CO crystal 
AL> using the coordinates given in 
AL> cst-www.nrl.navy.mul/lattice/struk.xmol/b21.pos. This has 14 CO 
AL> molecules. I used the format shown in example03 with the proper

no it doesn't! there are 4 CO molecules per unit 
cell. one in the corner and 3 in the faces. 

AL>  changes for the number of atoms and crystal structure.  
AL> Unfortunately the cal- culation does not converge.  Increasing the 

converge for self-consistency or for geometry?

AL> number of iterations and ecutwfc and ecutrho improved things a 
AL> little but the convergence is still about 10^-4 Ry. I am 
AL> particularly interested in the interactions between molecules so 
AL> this is not satis- factory.  Is there anything I can do to improve 
AL> the calculation to reach convergence to about 10^-7 Ry?

impossible to say without seeing your input file. 
i just did a quick test and it converges rather easily. 

AL>           Incidentally the calculated triple bond energy for a 
AL> single CO molecule is about 43 Ry while the literature value given 
AL> in texts is about 54 Ry.  Is this to be expected?

how did you determine this 43Ry value (or the 54Ry)? 

this would be equivalent to 27000 (or 33900) kcal/mol.
hardly realistic.

cheers,
   axel.


AL>           Hopefully someone has ideas about how to improve the convergence.



AL>                                                                   Amos
AL> Leffler
AL> 

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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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