[CPMD-list] cell parameter for some molecule

[Axel Kohlmeyer]

On Mon, 6 Jun 2005, sanjeev narooka wrote:

SN> Dear cpmd user,

dear sanjeev,

SN> could you please tell me how the cell parameter(cell
SN> dimension,symmetry,cutoff) was fixed  for the
SN> molecules like, CH4,C6H6,C2H8,C6H10, given in the
SN> cpmd-test directory. we can get the coordinates of
SN> atoms in these molecules using some graphical package
SN> but how we will set the cell parameter for a single
SN> molecules as it is given in cpmd-test directory.

well, basically the cell has to be large enough, that 
the molecules don't interact significantly with their
periodic images. so you could just take an orthorhombic 
cell and make it large enough. for nonpolar molecules,
that has to at least accomodate the charge distribution,
i.e. you have to add a couple of angstroms. if you have 
significant coulomb interactions, however, you should 
use a poisson solver (via setting SYMMETRY to 0) or else 
the box has to be too large until the interaction with
the periodic images become small enough. depending on 
which poisson solver you use, there are different 
requirements for the box size.

please see section 9.4 'Cell Size for Calculations with SYMMETRY 0'
in the cpmd manual for more details on that.

regards,
	axel.

SN> thanks
SN> sanjeev
SN> 
SN> 
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-- 

=======================================================================
Dr. 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.