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