===== U - function =====

This page shows how to calculate U - function. 
There is an simple example of calculation U -function for simple CH5N molecule below.
Here are input files.
 
----

CH5N.bas 
<code>
           7
   7      1.763266      0.773353     -0.259722
   6      0.474220      0.075638     -0.015967
   1      2.595372      0.133817     -0.297809
   1      2.017882      1.470682      0.480907
   1      0.229708     -0.640655     -0.836563
   1     -0.382559      0.790390      0.018898
   1      0.401767     -0.518300      0.926463
</code>
Calculation of U-function is turned on by iu = 1 in fireball.in.

fireball.in
<code>
&OPTION
basisfile = CH5N.bas 
icluster = 1
nstepf = 1
T_initial = 100.0
T_final = 100.0
sigmatol = 0.000000001
dt = 0.01
iquench = 0
max_scf_iterations = 200
iu = 1 
&END

&OUTPUT
iwrteigen = 1
iwrtxyz = 1
iwrtefermi = 1
&END
</code>
Next you need input file u.inp, where you have to write total number of electron. You can choose molecular orbitals for which you want to calculate U. There are 5 orbitals (6,7,8,10,22) in this case.
 u.inp
<code>
14     ! number of electrons
5      ! number of orbitals
6      ! states number 
7
8
10
22
</code>
The output file is called u_func.dat, where is written number of selected orbital in the first column and in the second one are u values.
<code>
           6   2.1590     
           7   2.3341     
           8   1.9742     
          10   1.9721    
          22   1.4045     
</code>