[Ifeffit] Crystallographic input for a structure having screw axis and co-ordinates shifted by certain amount

[madhusmita sahoo]

Dear Members,

I would like to know,

*Q1)* If there is a screw axis in the system, like the anatase phase of
TiO2, whose space group is I41/amd, how should I  enter that in the atoms
for feff calculation. Kindly tell me which  is the correct way.





When I entered 141(the space group number) and I41/amd, both generated
separate set of atomic positions.



 The older version of artemis, now the new one(latest version), both
generates different set of atomic position with the same crystallographic
input, i.e if I enter 141(or I41/amd) in place of space group and run
atoms, it generates two different set of co-ordinates in Artemis 0.8.012
and in Demeter 0.9.18. Ideally this should be same.





*Q2)* And when co-ordinates is shifted by certain amount, do we have to
 enter that in the shift vector as well as while defining the (x y z)  of a
particular site. Or I should just enter the shift vector and  define the
co-ordinates at its usual position ( I feel this one is correct).

 For example in anatase crystal structure  the co-ordinates are shifted  by
(0,0.25,-0.125).



So, the inputs can be

For shift vector 0,0.25,-0.125 and in place of sites Ti(core) x=0, y=0, z=
0 and O (scatterer) x= 0, y=0, z=0.20806

(Or) shift vector 0,0,0 and in place of sites Ti(core) x=0, y=0.25, z=
-0.125 and O (scatterer)  x= 0, y=0.25, z=0.08160 Ideally

both should  give the same result. But it gives different set of atomic
positions.



Even the same input in different version of artemis is yielding  different
results(in Artemis 0.8.012 and in Demeter 0.9.18) .

   I am confused about  how atoms work in artemis. I request you to
elucidate the correct method.



Warm wishes



Madhusmita
India
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