[Pw_forum] how to determine the value of U in a lda+U calculation

[duchl06]

dear pw users 
i want to do a calculation of V chain  absorb on a plane with DFT+U, but i donot know how to determine the value of U.
i found a PPT on the pwscf's media site, they said the value of U could be decised by a linear response approach.
but i really donot understand that like the input file resp_mat.in

 &input_mat
   ntyp = 1                           # number of types of atoms
   na(1) = 2                         # number of atoms per type
if we have different type atoms with different number, how we set these?

   nalfa = 5                         # number of perturbations (alpha)
   filepos = 'pos_sc1'          # file containing the atomic positions and the vectors of the unit cell
   back = 'neutral'               # to add a neutralizing “background”. see PRB 71 35105 (2005)
what does this mean
   filednda = 'file_sc1'        # file containing the names of the files dn*dat 
   n1 = 4
   n2 = 4                             # number of unit cells in each direction to construct the supercell for
   n3 = 4                             # extrapolation (in this case a 4x4x4 supercell of the SC one)
 &end

and they get the U is about 2.4ev for the iron, and they use 4.3ev for the FeO calculation, 
we always need a linear response approach to get the  U for every system 
or just a U for a certain pseudopotential (i mean if we get a U for V bulk, then if i could use this value for another system with V atom, like V2O3  etc.)
any suggestions or comment would be OK.
thanks 

Du
xiaguanying university
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