[CPMD-list] Warning

[Juerg Hutter]

Hi

this in fact touches several points.

1) Pseudopotential functions in CPMD are calculated on a
   radial grid in G-space and then used in spline interpolations.
   This speeds up variable cell calculations considerably.
   The maximum grid point is given by the cutoff. With the
   keyword
   SPLINE RANGE
     x.xx
   you can enlarge the grid to x.xx times the cutoff.
   
   BTW. the default number of spline points is 501. This is
   at the lower limit for accuracy. Especially if you have
   high cutoffs it is better to increase this value, e.g.
   SPLINE POINTS
     2500

2) In a variable cell calculation, CPMD uses a constant
   number of plane waves. Therefore if your cell
   contracts the cutoff increases, if the cell gets larger
   the cutoff decreases. So if you have the first case
   the spline interpolation needs points above the original
   cutoff and you get a warning.
   Depending on the amount of change in the cell you expect
   a value for the range of 2 - 5 is needed.

3) Comming back to the cunstant number of plane waves.
   If your cell gets larger the effective cutoff decreases.
   This may have very undesirable effects and it is better
   to define the plane waves on a box larger than any box
   you anticipate the simulation will reach.
   In order not to have to start with an unreasonable cell
   you can define the plane waves not with the actual box
   but with a reference cell, use the keyword

   REFERENCE CELL
     a b c xa xb xc

4) However, you really want to do a constant cutoff
   calculation, not a constant number of plane waves.
   For technical reasons this is not possible and in
   principle you should do the calculation at a high
   enough cutoff in order that the calculation is
   converged all along the simulation path (with the
   slight changes in cutoff).
   To avoid this very high cutoffs the group in Trieste
   came up with a method that allows to perform pseudo
   constant cutoff calculations.
   This method is implemented in CPMD and explained
   in this paper
   M. Bernasconi et al. J. Phys. Chem Solids 56 pp 501-505 (1995)

Hope this explains some of the issues of variable cell 
calculations with CPMD

regards

Juerg

----------------------------------------------------------
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 Mon, 3 Jun 2002, Martijn Zwijnenburg wrote:

> Hi,
> 
> I'm optimizing quartz (both ion positions and cell volume) using 
> CPMD and get the warning message: Warning! Spline region 
> smaller then maximum G-value. The optimization seems to 
> converge nicely but what does this warning mean / imply?
> 
> Cheers,
> Martijn
> -------------------------------------------------------------------------
> Martijn Zwijnenburg
> Lab. of Applied Organic Chemistry and Catalysis
> Delft University of Technology
> Julianalaan 136
> 2628 BL Delft
> The Netherlands
> Tel: 0031-(0)152782691
> Fax: 0031-(0)152784700
> e-mail: M.A.Zwijnenburg at tnw.tudelft.nl
> web page: http://come.to/tock
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