[Pw_forum] Energy of CO Crystal

[Amos Leffler]

Dear Forum,
       I have been trying to calculate the energy of a CO crystal using the

 coordinates given in cst-www.nrl.navy.mul/lattice/struk.xmol/b21.pos.
This has 14 CO molecules. I used the format shown in example03 with the
proper
 changes for the number of atoms and crystal structure.  Unfortunately the
cal-
culation does not converge.  Increasing the number of iterations and ecutwfc
and
ecutrho improved things a little but the convergence is still about 10^-4
Ry. I am
particularly interested in the interactions between molecules so this is not
satis-
factory.  Is there anything I can do to improve the calculation to reach
convergence
to about 10^-7 Ry?
          Incidentally the calculated triple bond energy for a single CO
molecule is about
43 Ry while the literature value given in texts is about 54 Ry.  Is this to
be expected?
          Hopefully someone has ideas about how to improve the convergence.
                                                                  Amos
Leffler
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