[CPMD-list] Problems optimizing monoclinic unit cell

[Axel Kohlmeyer]

>>> "EP" == Ewald Pauwels <ewald.pauwels at UGent.be> writes:

EP> Dear Axel,

dear ewald,

EP> First of all I would to thank you for your swift response.

you're welcome. 

>> this is to be expected. if you use CELL VECTORS, cpmd does not even 
>> try to detect cell symmetry and thus assumes a triclinic cell (and
>> cannot take advantage of the symmetry).
>> 
>> 
>> 
EP> In retrospect, this is indeed quite logical. So, whatever the original 
EP> symmetry, using CELL VECTORS would result in a "triclinic" cell in the 
EP> CPMD output?

exactly. you can see this from the source code. please have a look at
the files sysin.F around lines 415 and 820 and the file setsc.F 
around lines 30, 73, and especially 193 (line numbers correspond to
my current version, which may be a little different from the released
version).

>> as far as i understand it - please correct me anybody, if i am mistaken,
>> i'd really like to be certain about this - you have give the cell dimensions 
>> for the _conventional_ cell (but only the atom positions for the primitive cell). 
>> i know at least one more code, that uses the exact same conventions, so 
>> i am pretty sure, that this is intentionally so.
>> 
>> 
EP> Actually, I have given the cell dimensions for the _primitive_ unit cell 
EP> (which is monoclinic). This cell contains four glycine molecules, for 
EP> which I have specified all atom positions explicitly. So, I might have 
EP> misunderstood, but I don't think I have used a conventional cell in any way.

this is ok, but you simply need to give the cell dimensions
for the conventional cell corresponding to your primitive cell.

example: bulk ZnS. which is fcc.

in &SYSTEM i have:

SYMMETRY
 FCC
ANGSTROM
SCALE
CELL
 5.42 1.0 1.0 0.0 0.0 0.0

and specify the atom coordinates as (due to SCALE in fractional coordinates):

*Zn_MT_PBE_NLCC.psp KLEINMAN-BYLANDER NLCC
 LMAX=D LOC=S 
  1
      0.0 0.0 0.0

*S_MT_PBE.psp KLEINMAN-BYLANDER
 LMAX=D
  1
      0.2500000000      0.2500000000      0.2500000000


best regards,
     axel.


EP> Sincerely,
EP> Ewald.



EP> -- 
EP> Ewald Pauwels
EP> Centre for Molecular Modelling
EP> Laboratory of Theoretical Physics
EP> Ghent University
EP> Proeftuinstraat 86
EP> B-9000 Gent
EP> Belgium
EP> ewald.pauwels at UGent.be
EP> +32 9 264 65 76



--

=======================================================================
Axel Kohlmeyer       e-mail: axel.kohlmeyer at theochem.ruhr-uni-bochum.de
Lehrstuhl fuer Theoretische Chemie          Phone: ++49 (0)234/32-26673
Ruhr-Universitaet Bochum - NC 03/53         Fax:   ++49 (0)234/32-14045
D-44780 Bochum  http://www.theochem.ruhr-uni-bochum.de/~axel.kohlmeyer/
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.