Ocuupancy of a particular atom  indicate the probability of finding that atom at the corresponding atomic site. Occupancy = 1 (100%) would mean the site is completely occupied by the corresponding atom.  So, in the refinement one can vary this parameter (like other positional parameters) and try to look for the best fit. Any other value, e.g.,  0.9  would mean 10% vacancy at that atomic sites. However one should be careful about varying this parameter, since it can compensate the effect of other parameters, giving some unphysical structural model.  Also since oxygen is a low Z and hence low scatterer of x-rays the occupancy of such atoms from x-ray scattering would give rise large error.

For the stoichiometric compund you should enter the maximum occupancy for all the cationic and anionic sites that are present in the CIF file. In fullprof the way to insert the occupancy parameters is somewhat strange. Consider that for the occupancy you should take the mulpiplicity of the cationic site and refers it to the maximum occupancy of the space group of your sample. For example if Ca is in a multiplicity 2 site and the maximum multiplicity for the n. 204 space group is 48, the occupancy of Ca site to be inserted in fullprof is 2/48=0.0416. You should perform the same calculation for all the sites.

Thanks Dear Marcella Bini, but if we have 3 atoms in unit cell, for example Cu3, and its multiplicity is 6 and maximum multiplicity is 48 then whats is occupancy? 3.6/48=0.375 or 1.6/48=0.125? and what should be total occupancy of whole molecule? is it total 1?

I gather you have not exactly understood how to use the program. Imperative is that you know the structure -- since Fullprof is a refinement program, not one for solving the structure. Therefore, you must first of all know the space group (SG) that this compound of yours belongs to. Then the various atoms/ions (not molecules!) occupy the sites that are available in that very SG, and these have to be distributed in such a way that (taking full stoichiometry as the first approach) over the sites so that the composition of the phase is fulfilled. Therefore the ratios between the sums of multiplicites for each atom should be the same as in your compound (Ca:Cu:Ti:O= 1:3:4:12).

You first of all have to know the number of formula units to the cell (easily calculated from the formula and the density -- sometimes only guessed) which is an integer, Z. Suppose Z= 4, then you must have 4Ca, 12 Cu, 16 Ti and 48 O to be distributed over the available sites. How you end up with this distribution depends on the actual crystal structure -- there are a vast number of structures that have the same SG! (There are millions of structures but only 230 SGs.) 

Now you can follow the arguments given by Marcella. First you find out what the multiplicity is of the most general site in that very SG. Then you divide for each of the sites that are occupied its multiplicity with the maximum multiplicity and will arrive at various ratios. These are the numbers you should enter in the Fullprof program:

If the maximum multiplicity of the SG is 96 and your oxygen has a site multiplicity of 48, the number is 0.50; if your oxygen goes to two sites, each with multiplicity 12, then each of them will get 12/48= 0.25 (summing up to 0.50) in the program. It is essential that the atoms go to the correct sites, and the summing up of all of these site multiplicities must  give the total number of atoms to the cell (depending on the number of formula units, Z).

Now you can refine structural parameters and maybe later  refine occupancies. A special case is if two atoms share the same site (solid solution): Suppose that in your case calcium and copper both go to the same site; then the total occupancy is the one you arrive at if the site were occupied fully by only one element. Also such cases may be handled (so that the sum is kept) in the Fullprof program, but it is a little tricky how you enter your parameters to get it right!

Good luck!

Thanks Rolf Berger sir.

sir then how can we find bond angle and bond length from fullprof refinement?

You need to provide maximum and minimum distance between atoms, and also maximum angle between atoms in PCR file. There is a another inbuilt program in FullProf software, named "Bond-Str", here also you have to provide max. and min. dist. and angle with all structural parameters. After run the program, you will then get all possible bond distances and angles. 

Dr Suman Punia Use "Diamond" for bond angle and bond distances