simulating vibrational spectra in the solid state, help me!

Daria gali... at
Tue May 14 07:53:57 UTC 2013


I'm also working on vibrational spectra and I have found similar problem. 

I'm intersted in trying your NLCC pseudos. Can I ask you to send me the 
CP2K format so I can do some test?



Il giorno lunedì 13 maggio 2013 11:21:48 UTC+2, Florian Schiffmann ha 
> Hi,
> as always, the answer to your question and providing help is much easier 
> (and likely to happen) if input files are provided. 
> Cp2k is a bit special for vibrational spectra. The imaginary frequencies 
> are not necessarily connected to a badly converged geometry. Using GTH 
> pseudopotentials the density at the core decays to 0. This is a bit 
> problematic in the xc potentials as it leads to very sharp features. These 
> features can cause trouble in the FFT's (see Quickstep paper, smoothing). 
> Cutting a long story short, if you use the GTH pseudos (and an 
> insufficient CUTOFF) as they are this problem leads to a slight 
> translational invariance of the system as the result of the FFT depends on 
> the relative position of the atom with respect to the grid. This problem is 
> most obvious for 1st row transition metals and the elements B to Na. Also 
> it depends on the functional as the pseudos have different parameters (BLYP 
> pseudos are worst from my experience). I attach a graph with some PBE test 
> on this behaviour and cutoffs needed to achieve a good translational 
> invariance (<1E-5 Ha). Keep in mind using different functionals the 
> necessary  cutoffs can be different.
> At the moment there are two ways out either you have to use a hugh cutoff 
> (600+ depending on your elements) or using NLCC pseudos ( 
> ). The NLCC's are not yet in svn as they 
> are not 100% tested with every feature (energies and derivs are fine) and 
> they only exist for PBE and a limited number of elements (B - Cl). If you 
> are intersted in trying them I can send you the CP2K format. I will add 
> them to svn soon after testing is complete.
> regards
> Flo
> Re
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