[rodney]

I am having trouble understanding what it means for a organic compound to occupy a particular space group. I understand that space groups are cells that obey particular groups of symmetry and translation operations. What I don't understand is how we define the cell that a molecule exists in for a molecular crystal or how we identify the space group of a molecule.

Any thoughts or references would be much appreciated.

Thanks!

[AWK]

http://www.x-rayman.co.uk/xforum/viewtopic.php?f=11&t=1410

[rodney]

Ok. Still I have a few questions:

In a paper I'm reading, regarding crystal structure prediction, they attempt to determine the space group that a molecule occupies computationally. The methodology involves a space group search. When a low energy structure is found, then are the unit cell parameters simply compared to those of the different crystal structures and tested for their ability to have various symmetry operations preformed on them? Furthermore, how does molecular orientation within the cell effect the space group choice. For example, if I have cubic cell, with a complicated randomly oriented molecule in the center, what determines what space group it occupies? Theoretically, couldn't I place the crystal in any lattice system and find a low energy configuration?

Again, tips and references would be much appreciated.

Thanks

[AWK]

http://en.wikipedia.org/wiki/Crystal_structure_prediction
and search with google for: crystal structure prediction

[rodney]

In case anyone else reads this, the clearest description of what it means for a molecule to occupy a certain space group that I have found is the following:

A space group is a closed ensemble of symmetry operations such that the product of any two operations is still a group operation. Therefore for a molecule to occupy a space group, a molecule must exist in a periodic lattice, in such a way that the molecular coordinates of any particular molecule in the lattice, can be operated on using a defined set of symmetry operations, such that you end up with the molecular coordinates of any other molecule in the crystal. The set of operations that allow such a mapping to take place, define the space group of the molecule.