Recommendation on the use of counter ions or general functional/basis set combination

[NORMAN GEIST]

Dear experts,

while moving from CPMD to the lovely CP2k we stumbled into the problem, 
that the use of counter ions e.g. Na+ is regulating our cutoff. The rule of 
thumb says one should take approx. the highest exponent of the basis sets 
in the system multiplied by 40 to get a good energy conservation. This 
results in a cutoff of 680 for the Na GTH BLYP. I've seen that for some 
functionals there are also weaker versions of the basis sets or pseudo 
potentials with less electrons, but it seems that for no functional there's 
a really complete set of pseuo potentials and basis sets around.

So currently we have the following questions to clear up our confusion:

1. Should we use BLYP as our preferred functional, what are you guys using 
for MD?
2. Is PADE as bad as we think, assuming it's only local density 
approximation?
3. Should we use basis sets and pseuo potentials that do not fit in the 
number of electrons?
4. What is the "usual" functional to use for simple biological systems?

Thank you all

Norman Geist
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