Dear friends, For ur question, I feel better to you details.
simply, I quoted the text from International Crystallographic Volume A for you ref. "An asymmetric unit of a space group is a (simply connected) smallest closed part of space from which, by application of all symmetry operation(hopefully you what is symmetry operation, please refer to crystallographic textbook) of space group, then the whole space of unit cell is filled with atoms. This implies that mirror planes and rotation axes must form boundary planes and boundary edges of asymmetric unit. A twofold rotation axis may bisect a boundary plane. centres of inversion must either form vertices of the asymmetric unit or be located at the midpoints of boundary edges." Anyway, the asymmetric unit contains all the necessary information for the completion of crystal structure. Mathematically, asymmetric unit is called as fundamental region or fundamental domain. Therefore, what you have to put in GSAS atom box, is the atomic information of asymmetric unit, and then leading to construction the whole crystal structure, generating Bragg scattering intensity therewith. Given how to determine the input of asymmetric unit info, there are two ways to do so, one of them is extract info from crystal database, however it depends, if you got license to get in, alternatively, you should have to think it over, and do it by yourself. Whatever you use, you should have a perusal of the book i suggest above first. with best wishes, ----- Original Message ----- From: <[EMAIL PROTECTED]> To: "Rietveld Mailing List" <[EMAIL PROTECTED]> Sent: Wednesday, November 03, 2004 3:53 PM > > Dear All, > One thing is sufferring me a lot. I don't have much idea about space groups. > In case of giving positions of atoms in GSAS, so far I understand when I am giving the space group information, I should not have to give the positions of all the atoms. It is understood from the space group information. > > For example: For fcc Nickel with space group 225, I have to only give the position of one atom (0,0,0).Not the (0.5,0.5,0.0) ... > > But I can't understand, if know the space group of a system, how can I decide what position and how many atom position I should insert in GSAS. > > I will be grateful, if anybody help me in this regard. > > Regards, > Apu > > > /_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/ > Apu Sarkar > Research Fellow > Variable Energy Cyclotron Centre > Kolkata 700 064 > phone: 91-33-2337-1230 (extn. 3190) > Fax: 91-33-2334-6871 > INDIA > /_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/ > > > >
