simulating vibrational spectra in the solid state, help me!
gali... at gmail.com
Fri May 17 16:57:24 CEST 2013
thanks for the file and for the advice
Il giorno mercoledì 15 maggio 2013 14:16:54 UTC+2, Florian Schiffmann ha
> Hi Daria,
> I attached the NLCC pseudos. As said before they are only optimized for
> PBE and there is no warranty that they work for anything else (e.g. PBEsol,
> Previously I forgot to mention two other option to overcome that problem:
> 1) Use SMOOTING (splines) in the XC_GRID section. Personally I wouldn't
> recommend that option. It solves the riplle problem but has a hugh effect
> on the total energy and converges slower to the PW basis set limit.
> Furthermore I am not 100% whether the error due to smoothing is systematic
> or not.
> 2) Use the USE_FINER_GRID option in XC_GRID. This will be expensive for
> calculations dominated by FFT anyway. This uses a grid with 4*CUTOFF for
> the xc part. I am about to commit the option to specify the finer grid by
> the user but it isn't quite ready. With this option (PBE case) you should
> get away with a CUTOFF of 400.
> In my oppinion the best options are NLCC's or the finer grid. NLCC's have
> the problem that they are hard to fit and there is no hope that there will
> be an extended set in the forseeable future. Therefore the FINER GRID
> provides an option for all calculations and has several advatages to the
> other options:
> 1) It really improves accuracy unlike smoothing
> 2) forces and 2nd derivatives are better compared to an increase of the
> overall cutoff. This can be understood as the energy ripples are mainly due
> to the xc bit and will be accurate to the limit of the Finer grid. The
> potential neverthelees gets mapped back on the total grid and thus the
> ripples appear on a larger grid spacing than in case of high total cutoffs.
> If you take now the numerical derivative of the grid 'ripples' the larger
> spacing significantly reduces the force contribution.
> Hope that helps
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