:orphan:
 

The :math:`UF_6` anion - another approach
=========================================

Different and more robust solution for obtaining the SCF convergence of the :math:`UF_6` anion is provided here.
It is based on the damping of the Fock matrix from SCF iteration to iteration.

Starting from neutral :math:`UF_6`
----------------------------------
In the first step we follow the previous approach of starting from the closed-shell (neutral) :math:`UF_6` system.
(We also may utilize a shift in the virtual space to reduce mixing between occupied and virtual spinors.) In any case, we
get smooth convergence with the default DIIS method. Files to download are :download:`UF6neutral.inp`, :download:`UF6.mol`,
:download:`cc-pVDZ-PP` and :download:`ECP60MDF`: ::

 pam --noarch --inp=UF6neutral.inp  --mol=UF6.mol --get "DFCOEF=DFCOEF.UF6neutral"

The :math:`UF_6` anion from neutral
-----------------------------------
In the next step we compare two calculations starting from neutral molecular spinors of UF6 (the file DFCOEF.UF6neutral). 
One calculation uses the DIIS method (not shown here) and the other uses the damping with two damping factors of 0.90 and 0.50. 
(File to download is :download:`UF6_anion_scfdamp.inp`): ::

 pam --noarch --inp=UF6_anion_scfdamp.inp --mol=UF6.mol --put "DFCOEF.UF6neutral=DFCOEF" --get "DFCOEF=DFCOEF.UF6anion_damp_pass1" --replace dampfactor=0.9
 pam --noarch --inp=UF6_anion_scfdamp.inp --mol=UF6.mol --put "DFCOEF.UF6anion_damp_pass1=DFCOEF" --get "DFCOEF=DFCOEF.UF6anion_damp_pass2" --replace dampfactor=0.5

Concerning the DIIS speedup method, the SCF iterations initially converge to ca :math:`10^{-5}` but then began to diverge
and after about 150 iterations the error gradient of the Fock matrix is still around :math:`10^{-5}`.

However, with the damping convergence speedup the calculation converges to ca :math:`10^{-8}` after about 50 iterations.
The damping scheme uses more of the input Fock matrix that the output Fock matrix.
Concerning the damping factor, the lower than 1 it is, we should get faster convergence. However, when the factor is getting too small,
iterations diverge. The closer we are to the converged states, the smaller the damping factor can be.

If we begin calculations from the converged solutions from the damping, the DIIS iterations converge 
rapidly after few iterations. But this is only with the deactivated active-active shells interaction
(new file to download is :download:`UF6anion2.inp`): ::

 pam --noarch --inp=UF6anion2.inp --mol=UF6.mol --put "DFCOEF.UF6anion_damp_pass2=DFCOEF" --get "DFCOEF=DFCOEF.UF6anion_diis"

The :math:`UF_6` anion with OPENFAC=1
-------------------------------------

Unfortunately, with the factor OPENFAC=1 (what is the keyword :ref:`SCF_.OPENFACTOR`) we were not able to get the convergence,
neither with the default DIIS, nor with damping factor (to download, :download:`UF6anion.inp` and :download:`UF6anion3.inp`): ::

  pam --noarch --inp=UF6anion.inp --mol=UF6.mol --put "DFCOEF.UF6anion_diis=DFCOEF"  
  pam --noarch --inp=UF6anion3.inp --mol=UF6.mol --put "DFCOEF.UF6anion_diis=DFCOEF"  --replace dampfactor=0.8

More effort is required to obtain converged molecular spinors of the :math:`UF_6` anion 
with the active-active open-shell interaction (that is OPENFAC=1). We leave this task to the reader.