[Pw_forum] CO and N2 Crystals

Nicola Marzari marzari at MIT.EDU
Sun May 6 18:46:09 CEST 2007

Dear Amos,

ibrav specifies the 3-dimensional repetition pattern (i.e. the Bravais 
lattice) that you apply to the individual atoms you specify.

E.g., an infinite fcc crystal of Al can be described as one Al atom
repeated according to the translational symmetry of a fcc Bravais
lattive, or as 4 atoms repeated in a simple cubic Bravais
lattice, or in many other ways. One atom repeated fcc is the least
expensive approach.

A crystal of CO would typically have 2 atoms specified, and the
ibrav of the crystal you think CO has. Of course molecular crystals can
have unit cells that contain more than one inequivalent molecule - so 
you would have to specify more than 2 atoms. An isolated CO molecule 
would be described by a crystal with a lattice parameter large enought 
where the CO - CO interactions have become negligible.

Having said this, this is a question that is not appropriate to the
forum - it's really about crystals et al. Ashcroft Mermin chap 4-7 could
be a good start, or the material in


Amos Leffler wrote:
> Dear Forum,
>          I would like to calculate the properties of the CO and N2 
> structures.  Example03 calculates the scf for CO but does nothing about 
> the crystal.  The problem is whether to use ibrav=0 to place each atom 
> or to use CO units with ibrav=2.  Little is said in the Users Guide 
> about either of these approaches and I have looked at some of Forum 
> records but haven't found anything about either of these methods.  
> Hopefully someone can point me to references or suggestions.  There 
> appears to be nothing about using polyatom elements for crystalline 
> structure but this would be a very useful ability since there are many 
> cases where it could be applied.
>                     Amos Leffler
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Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu

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