simulating vibrational spectra in the solid state, help me!

[Florian Schiffmann]

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 ( 
http://arxiv.org/abs/1212.6011 ). 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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