simulating vibrational spectra in the solid state, help me!

[Daria]

thanks for the file and for the advice

Daria

Il giorno mercoledì 15 maggio 2013 14:16:54 UTC+2, Florian Schiffmann ha 
scritto:
>
> 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, 
> PBE0,...). 
> 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
> Flo
>
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