[CPMD-list] Size of cell for SURFACE/POLYMER ?

[Juerg Hutter]

Hi

here some information on the keywords for isolated
system calculations.
There is some information in the manual and a discussion
in the review
 -  D. Marx and J. Hutter
       "Ab-initio Molecular Dynamics: Theory and Implementation",
       Modern Methods and Algorithms in Quantum Chemistry,
       Forschungzentrum Juelich, NIC Series, vol. 1, (2000).

       www.theochem.ruhr-uni-bochum.de/research/marx/marx.pdf

All methods require that the charge density is zero at
the border of the box. For normal systems this means that
about 3 Angstrom space between the outermost atoms and
the box should be enough. However, for some systems and for
high accuracy this may not be enough.
Some methods have additional requirements I will discuss later.

ISOLATED SYSTEM keyword has only an effect on the
calculation of the degrees of freedom (3N-6 vs. 3N-3
for periodic systems).

CENTER MOLECULE ON/OFF: The main purpose of this is to
center the molecule (center of mass) in the box. This
is needed for the HOCKNEY Poisson solver. This solver
gives wrong results if the charge density is not
centered in the computational box. All other solvers
behave like the periodic counterpart, i.e. the relative
position of the charge density and the box are not
important.

Isolated systems (i.e. decoupling of the electrostatic
images in the Poisson solver) are initialized with
SYMMETRY
   0
The box is assumed to be orthorhombic.
Poisson solvers available are
POISSON SOLVER {HOCKNEY,TUCKERMAN,MORTENSEN}

HOCKNEY Method:
  - molecule has to be in the center of the box
  - box size molecule + 3 A border
  - expensive for very small systems
  - not available for some response calculations
  - SURFACE/POLYMER available but not tested
    use at own risk

TUCKERMAN Method
  - box size : molecule + 3 A border AND 2*size of
               charge distribution
  - expensive for large systems, smaller boxes
    might be used without loosing too much accuracy
    SURFACE/POLYMER not available

MORTENSEN Method
  - same as TUCKERMAN, but using analytic formulas
    made possible by using special boundary conditions
    (sphere, rod)
  - SURFACE/POLYMER available, should be save to use

Remember: for many systems using a large enough cell and
          periodic boundary conditions is also an option!



----------------------------------------------------------
Juerg Hutter                   Phone : ++41 1 635 4491
Physical Chemistry Institute   FAX   : ++41 1 635 6838
University of Zurich           E-mail: hutter at pci.unizh.ch
Winterthurerstrasse 190
CH-8057 Zurich, Switzerland
----------------------------------------------------------


On Thu, 15 Aug 2002 jarek at elrond.chem.uni.wroc.pl wrote:

> Dear CPMD users,
>
> I have a question related to the latest discussion on the 2D calculations
> in CPMD. What is the required cell length in the "non-periodic dimensions"
> for SURFACE, POLYMER or, while we are at it, ISOLATED MOLECULE? The
> standard supercell approach would require the sufficiently large length
> for decoupling of the periodic images, but what about the options listed
> above? Should we include, say, 3 or 4 A of "free space" from the most
> protruding atoms? If anyone has got either a theoretical explanation or
> the empirical rule of thumb, I would be happy for hearing it.
>
> And, of course, happy summer holidays to you all!
>
> Jaroslaw Panek
> Faculty of Chemistry, University of Wroclaw
> ul. F. Joliot-Curie 14, 50-383 Wroclaw, Poland
>
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