[CP2K:757] Parameter choice for GAPW
Marcella Iannuzzi
marcella.ia... at psi.ch
Tue Mar 11 00:35:53 UTC 2008
Hi Axel,
Playing with the GAPW parameters requires some insight in the formalism.
I will try to summarize briefly the role of the most relevant GAPW
parameters.
You are right they are grouped in DFT%QS and in KIND
In QS section
- EPSFIT: this parameter is used to determine which primitive
Gaussians are considered hard and which one soft. With the default
values exponents >= 4 are considered to be hard.
- EPSISO, EPS_SVD and MAX_RAD_LOCAL are parameters used in the
construction of the GAPW projectors. Playing with them might help in
difficult cases that turn out to have instabilities. Otherwise they
can be safely left to default values
- in GAPW local compensation densities are needed in order to compute
correctly the hartree contributions that involve the local densities.
In the original scheme we have a hard compensation density, expanded
in Gaussians with fixed exponent ALPHA0_HARD, and a soft compensation
density that has the same multipole expansion as the hard one and it
is use also on the fft. Having two different compensation densities
introduces some additional terms in the expression of the total
energy. These terms are rather expensive to be computed because they
involve three center neighbor lists. By default ALPHA0_HARD is set to
10
and ALPHA0_SOFT=ALPHA0_HARD, so that all the complex 3-center terms
disappear. Since we noticed very small differences by having or not
having these terms, by default we set equal hard and soft exponents ?
IF FULL_GAPW is set to true, all the terms are computed. If the
exponents are not given from input, they are automatically set to
meaningful values.
- LMAXN0 and LMAXN1 are maximum angular momenta used in the expansion
of compensation charge and local charges. The maximum possible value
is 2 times the maximum angular momentum present in the basis set.
Having LMAXN0 or LMAXN1 smaller than this can improve the performance,
but might have an effect on the accuracy.
-QUADRATURE this variable defines the algorithm used to assign the
points on the local radial grid
in KIND
HARD_EXP_RADIUS is the radius of the region around the atom where the
local densities are defined. The spherical regions centered on
neighboring atoms do not have to overlap, therefore the radius cannot
be too extended..
LEBEDEV_GRID RADIAL_GRID are parameters that determine se size of the
local grids, The optimal choice depends on the system and on the
maximum angular momentum that need to be collocated. For not too
complex systems one can use about 100 points on the radial grids, and
50 on the angular.
Other useful info:
- GAPW does not need a large PW cut-off, since the hard density is
computed on the local atomic grids.
- Comparing GAPW and Gaussian calculations for small molecules you
should find differences of a few microhartree. Due to the definition
of the local densities, GAPW improves systematically by improving the
basis set
- if you use all electron and OT, convergence is significantly
improved by setting the preconditioner to FULL_ALL
I am available for further clarifications on the method and its
implementation.
ciao
Marcella
On 10 Mar 2008, at 18:35, Axel wrote:
>
> hi!
>
> i'd appreciate it, if somebody could shed some light on how to
> justify
> parameter choices for GAPW calculations. the input reference mostly
> contains statements of the kind XXX sets the value for 'xxx', which
> are
> probably of most use to the person who knows the implemention very
> well
> (which i obviously don't :-) ).
>
> from looking through the regtests and the input reference it seems to
> me
> that ALHPA0_HARD and ALHPA0_SOFT, LMAXN0, LMAXN1 in %FORCE_EVAL%DFT
> %QS
> seem to be the most important global options, and that LEBEDEV_GRID,
> RADIAL_GRID, and HARD_EXP_RADIUS in %FORCE_EVAL%SUBSYS%KIND seem to be
> the most
> important per atom type parameters.
>
> are there any do's and don'ts associated with those parameters, e.g.
> what needs to
> be changed when going from a bulk water system to a compound with
> "heavy" elements?
> which of those parameters are related to higher angular momentum and
> thus need to
> be increased or decreased? can the HARD_EXP_RADIUS and its sibling
> RHO0_EXP_RADIUS
> be understood in a similar way as the ultrasoft cutoff radius for
> USPPs?
>
> thanks in advance,
> axel.
>
> >
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