[Pw_forum] CO and N2 Crystals

Amos Leffler amos at errno.com
Sun May 6 22:03:42 CEST 2007

Dear Nicola,
       My thanks for such a rapid reply especially on Sunday!
I will look at the download as soon as possible.
         Amos Leffler
P.S. I enjoyed your video courses.

Nicola Marzari wrote:
> 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
> http://ocw.mit.edu/OcwWeb/Materials-Science-and-Engineering/3-320Spring-2005/CourseHome/index.htm 
>                 nicola
> 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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