At the C-terminus of a chain of amino-acid residues, there is a “modification” so that the C-O becomes a carbonyl, i.e. an extra (terminal) oxygen (OXT) needs to be added. This atom is added so that it is in the plane of the C\alpha, C and O atoms of the residue.
Scripting usage:
(add-OXT-to-residue imol residue-number insertion-code
chain-id) 1,
where insertion-code is typically "".
Note, in order to place OXT, the N, CA, C and O atoms must be present in the residue - if (for example) the existing carbonyl oxygen atom is called “OE1” then this function will not work.