[Wien] Force optimization of a spinel compound

[shamik chakrabarti]

Dear wien2k users,

          We have optimized atomic coordinates of a spinel cubic compound
containing 56 atoms/cell. The most important variation we have seen between
experimental structural coordinates & DFT (GGA+U) optimized coordinates is
that.

the oxygen coordinate changes from (x,x,x) configuration to (x,y,z)
configuration as we have not constrained the oxygen coordinates during
optimization.

Although, this change has generated a structure having very small forces (<
1 mRy/Bohr) per atoms, the change in oxygen coordinates are significant
(difference between x & z is 0.021) enough to change the relative
intensities of different peaks of XRD patterns.

Now my query is (before starting the experimental simulation by myself
again!!) whether there is a way to constrain atomic coordinates such that
it remain constrained to (x,x,x) configuration while changing the value of
x?

& also whether such a constrained way should be adopted or should we accept
the DFT optimized structure (a bit different from experimental one!!) as it
is for further calculation?

Looking forward to your opinions & suggestions.

Thanks in advance,

with regards,

-- 
Shamik Chakrabarti
Senior Research Fellow
Dept. of Physics & Meteorology
Material Processing & Solid State Ionics Lab
IIT Kharagpur
Kharagpur 721302
INDIA
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