Treatment of merohedral twinning

[@DilipKumar-it6gm]

Hello Sir, Thanks for making videos on this topic. your video is too supportive of our work. There are some queries, a) if we collect data by mar345 detector, sometimes on data reduction process we didn't find *Rint* values or sometimes higher *I/sigma* which create problems in data solving. Is there a necessity to have Rint value because sometime after full data is solved even the R factor is left so high? b) can you provide the download link for SHELXTL to generate the *ins* file? c) Our data, most of the time pick 1 centrosymmetric and 1 noncentrosymmetric unit at a time to solve *higher nuclearity clusters*, which is difficult to solve for us. May you make a video to solve this type of data?

[@DrKuldeepMahiya]

Thank you for the feedback. Two major criteria to be taken into consideration: the I/σ measure the strength of signal, and the merging R value Rint (or Rsigma) of all data within a shell. Generally, I/σ become smaller with higher resolution, while the Rint values grow. What now are the minimum values for I/σ and the maximum values for Rint that distinguish ‘data’ from ‘noise’? Many crystallographers agree that data with overall values of I/σ ≤ 2.0 and/or Rint ≥ 0.45 throughout a certain resolution shell are to be considered noise. Value of Rint zero is possible. SHELXL automatically rejects systematically absent reflections. The sorting and merging of the reflection data is controlled by the MERG instruction. Usually MERG 2 (the default) will be suitable for small molecules: equivalent reflections are merged and their indices converted to standard symmetry equivalents, but Friedel opposites are not merged in non-centrosymmetric space groups. High R factors even with low Rint may be due to unresolved disorder, twinning, wrong space group etc. SHELXTL is now available with APEX software by Bruker which is a licenced software. You can ask for a demo licence for six months. Generally it is good practice to solve data in a centrosymmetric space group but non centrosymmetric space group is a better choice in casre of pseudosymmetry. I am going to make my next video on pseudosymmetry sooon. Hope it will help.