QM free energy calculations

[Vladimir F]

Hello!
I'm trying to compute hydration free energy of Na+ using AIMD & TI & force 
mixing. The classical computation with SPC water has been successful (400 
kJ/mol against ~414 reference), but the QM one (PADE functional & DZVP 
basis set) has resulted in a as twice as large value 800 kJ/mol. Initial 
cells (water and Na+ in water) were pre-equilibrated (MM and 20 ps QM). 
Productive runs were 1-5 ps before I aborted them. The difference seems to 
be too large to attribute it to short duration only. Please explain, is it 
really so (and I should continue) or is it a mistake in input? Maybe I 
 should apply some corrections to QM results? Also what the "DERIVATIVE 
TOTAL FREE ENERGY" output values mean (I used "DERIVATIVE OF TOTAL ENERGY [ 
PARAMETER (k) ]" values)?
Input and output files for k=0.5 and dU/dk plots are attached. Gray points 
are extrapolated ones.
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