Dear Sir/Madam, Sorry to bother. I tried to use make_ndx to generate an index file so that it will select c-alpha and some residues range from 44-53 and 82-92. But I tried 3 & r 44-53 82-92 and ( 3 & r 44-53) | (3 & r 82-92) but neither of them worked. Is there someway I can accomplish this? Best Wishes, Li Su