From:   TOPHAM::PD           "Powder Diffraction Course" 26-NOV-2002 15:05:03.07
To:     SMTP%"nkourkou@cc.uoi.gr"
CC:     PD
Subj:   RE: REQ: Assignment 2 (&Enrolment Papers delay)

Dear Nikos,

>I am writing to you in order to inform you that unfortunately I haven't
>received any enrolment papers so far...
>Of course, I will let you know as soon as I receive them hoping that there
>will be no problem with the Registry due to this delay.

I have forwarded this on to the course administrator to chase up. The college
Registry has given us problems for years... One student from the USA described
his experience as like dealing with the "quill & ink" era! My apologies,
however, we have no choice but to work with this official section of the
college administration.

>Moreover, I would like to ask something about "release 5" of "assignment 2".
>The last questions include the following symmetry operators : 1/2-y, 1/2-x.
>Do they mean "translation on the negative axis-y (or x)" or "backwards
>translation"? I cannot find any clear answer on this...

Please can you be more specific? I will try to answer what I think you are
asking anyway, so please come back to me if I have missed the point.

Firstly, let me say that the last few problems of Q5 of assignment 2
are designed to make you think: specifically viii to x are not directly
in the course text! (You may even have to guess...)

Secondly, an expression of, e.g., the type ..., 1/2-y, ... means that the new y
coordinate is 0.5 minus the old value. So if the atom on x,y,z has coordinates,
say, ..., 0.321, ... (I am leaving out x and z here), then the symmetry
related atom will have coordinates ..., 0.5-0.321, ... = ..., 0.179, ...
The -y does not refer to the "negative axis-y" - it simply says make the
old y value negative (or if a contant e.g. 1/2 is present, then subtract
it from that constant).

Finally, model answers to the questions will be provided, which will explain
all of them (and also provide a few general rules that are worth learning too).

I am not sure if this helps?

Many thanks,

Jeremy Karl.

------------------------------------------------------------------------------
                       Internet Powder Diffraction Course
                  URL: http://pd.cryst.bbk.ac.uk/pd/welcome.htm
School of Crystallography, Birkbeck College, Malet Street, London WC1E 7HX, UK
------------------------------------------------------------------------------

Back