[Pw_forum] cohesive energy of C60

[Paolo Giannozzi]

On Jan 12, 2007, at 21:30 , Jose Manuel Napoles Duarte wrote:

> I am trying to do a test pwscf calculation of fullerene C60 in a FCC
> bravais lattice. I have the reference of Saito and Oshiyama (PRL 66,
> 2637 (1991)) and that of Troullier and Martins (PRB 46,1754 (1992))
> where both groups found a cohesive energy of 1.6 eV. However in a
> preliminar result I have found [...] a cohesive energy of  
> 0.81410756 eV

what a waste of digits, for a value

> which I guess is about 1/2 of the energy reported in the references
> given above

and is physically meaningless (solid C60 is bonded by van der Waals
forces). Anyway: try to make a calculation as similar as possible to
the one of Troullier and Martins: same cell, same k-points, no
smearing (C60 is an insulator), pseudopotentials as similar as possible
(ultrasoft are nice because they allow you to use a small cutoff, but
if you don't, they are just a hassle). First of all, however, check that
the calculation for the "isolated" molecule is performed using exactly
the same cutoff, same pseudopotential, etc, and that the cell is
large enough.

Years ago I made a large number of calculations on C60 systems.
I don't remember having checked the cohesive energy, but everything
else I got was quite the same as in the TM paper

Paolo
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Paolo Giannozzi, Democritos and University of Udine, Italy