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

duchl06 duchl06 at 163.com
Mon Apr 5 17:09:08 CEST 2010

```Dear Matteo
thanks for your quick reply, so another question as i mentioned in the former email.
if we have different atoms with different number, how we set the value of the keyword below
>> ntyp = 1 # number of types of atoms
>> na(1) = 2 # number of atoms per type
or is it only relate to the atom with a nonzero U value.

best regards!
du

>
>Dear Du,
>
>the input you found below is to extract the U values from a series of
>scf calculations.
>You can give a look (besides the reference cited below) to the tutorials
>you find in this webpage:
>http://www.quantum-espresso.org/wiki/index.php/QESB09.
>Unfrtunately the value of U is generally not portable and changes
>depending on the invironment of the atom in the
>specific crystal you are considering. the one you cite is a good
>example: Fe in bulk Fe and in FeO.
>So in principles we need to recompute it each time.
>
>hope this helps.
>
>Matteo
>
>
>duchl06 wrote:
>> 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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>>
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