If it is the same, which is the right nomenclature H3 or R3?
I would like to know the advantages from a crystallographic point of view of using an hexagonal representation?
Any other explanation concerning this topic would be very helpful.
The space group R3 and H3 are not strictly the same. Therefore R3 and H3 have been given different space group number in the International Tables of Crystallography. R3 has number 146 whereas H3 has the number 143. R3 is rhombohedral whereas H3 is H centered trigonal. Please consult International Tables and try to understand the symmetry elements of these two space groups instead of using back-box softwares.
Hola Rossana, Que bola?
I am sure that you will get a detailed answer describing the theories of why it should be R3 or H3 or the subtle diffrences between the two. The truth is that it will not change your work flow. The generally process as R3 in HKL (I am not sure about mosfilm or D*trek) and if you use the phenix suite of software, then the R3 designation will remain. CCP4 suite of software will change to H3. But you can use the same pdb and mtz files back and forth.
The space group R3 and H3 are not strictly the same. Therefore R3 and H3 have been given different space group number in the International Tables of Crystallography. R3 has number 146 whereas H3 has the number 143. R3 is rhombohedral whereas H3 is H centered trigonal. Please consult International Tables and try to understand the symmetry elements of these two space groups instead of using back-box softwares.
You may be confusing the R3 with P3.
P3 and H3 are the same space group (#143 as Tapan pointed out).
The difference is the choice of the unit cell.
In the P3 notation, the unit cell is the primitive unit cell.
In the H3 notation, a hexagonal unit cell is used which is 3 times larger.
When doing crystallography the old-fashioned way, one peak at a time aligned more or less manually, the hexagonal notation is sometimes easier to interpret. Both notations are correct and completely equivalent.
See e.g.
http://img.chem.ucl.ac.uk/sgp/large/sgp.htm
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