[sdpd] Re: One question about molecule location methods

[MartinUlrich_Schmidt... @de.clariant.com]

Dear Armel, dear SDPD member,


You asked, what to do, if it is not possible to insert a fragment
in a general position as a whole.
If this is a space group problem, let me try to answer your question:

It is known, that there are some cases, where it is not possible
to insert a fragment in a general position as a whole, because
there is no corresponding subgroup of the assumed space group
This is the case in e.g. P4 with the molecule situated on
the twofold axis. The only translationengleiche subgroup is P2,
in which the molecule is still on the 2-fold axis. There are 3
possibitilies
to circumvent this problem:
1) Choose a non-translationengleiche subgroup, which does
not contain the molecular symmetry element (in the case of P4, Z=2:
choose P4, Z=4, a'=a+b, b'=a-b)
2) Go a further step down in symmetry (in this case to P1). But I do not
recommend this, since it unnecessarily increases the number of
independent molecules per unit cell.
3) Make the calculation by explicitly giving the symmetry operations,
although the symmetry operations do mathematically not form a group.
In the case of P4: give only x,y,z and y,-x,z.

I hope this fits your question.

Martin U. Schmidt

- - - - -
Martin U. Schmidt
Clariant GmbH
Pigment Technology Research
G834
D-65926 Frankfurt am Main
Tel.: +49 69 305 13426
Fax: +49 69 331749
e-mail martinulrich.schmidt... @clariant.com