W.R. fred... at
Tue Apr 12 10:31:45 UTC 2016

Hi all,
I want to use DFT within cp2k to get an estimate of the energy of the 
charge transfer state of 2 molecules. Hence, I was looking for a method 
that forces molecule A to a charge of +1 and molecule B to a charge of -1. 
So far, I have been playing around with BECKE_RESTRAINT and DDAPC_RESTRAINT 
within the DFT->QS section and also within the PROPERTIES->ET_COUPLING 
section (not simultaneously though).

I have a couple of questions regarding my results so far:

1) What do I specify with the target value? The total charge of a fragment? 
(and is it defined the same for BECKE and DDAPC?)

2) I get completely different values in the output for "Final target value 
of constraint A:" for Becke and DDAPC. For Becke I was assuming that the 
value should be (number of valence electrons +/- 1) for fragment A/B, i.e. 
in my case 83 and 85. (I have obtained these values, but this also results 
in a far-too-high coupling value, so there is probably something wrong.) 
With ddapc on the other hand, I get two completely different, and mutually 
hardly varying values for the "Final target value of constraint A" and of 
constraint B, both between 0 and 1. So my question is what these final 
target values tell me, what do they represent, and what/why is the 
difference between the final target values of the two methods?

3) Am I right if I say that I should tweak the strength-variable till I 
obtain a correct "final target value"?

4) Can one beforehand tell if one of the sections (QS vs ET_coupling) is 
more appropriate for this case than the other section?
(I don't need the coupling info, but with QS's Becke-restraint, I can only 
apply one restraint; I guess that this can be countered by setting the 
overall charge to 0 though)

5) There is also Mulliken and S2_restraint in the QS-section. If anyone 
knows if one of the four restraint-methods is (most) 
appropriate/inappropriate or you have good/bad experiences with one of 
them, please let me know. 

If you have the answer or experience to one or more of the questions, 
please let me know, would really help me a lot!

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