mol_fix_origin_$maxid in rigidbody separate.cns can lead to failures of the rigidbody docking

[amjjbonvin]

Module/Workflow/Library affected

rigidbody module, separate.cns script

Expected behavior

When mol_fix_origin_1 = true the mol_fix_origin_$maxid used in separate.cns should have a valid value between 1 and the number of molecules defined.

Actual behavior

Currently, maxid is initialised to 0 and it can happen in the next code snippet from separate.cns that maxid remains to 0 and this will cause the last line of the code in the snippet to fail because mol_fix_origin_0 is not defined (it starts at 1).

       while ($sum_random > 0) loop placemol
            evaluate ($maximum = 0)
            evaluate ($maxid = 0)
            evaluate ($ncount = 0)

            ! Select molecule with largest random number
            while ($ncount < $data.ncomponents) loop selectmol
                evaluate ($ncount = $ncount + 1)
                if ($random_$ncount > $maximum) then
                    evaluate ($maximum = $random_$ncount)
                    evaluate ($maxid = $ncount)
                end if
            end loop selectmol

            if ($mol_fix_origin_$maxid eq false) then

Steps to reproduce the behavior

Difficult to reproduce as it has a random aspect. The problem was observed by a user:

https://ask.bioexcel.eu/t/rigid-body-step-fails-with-100-output-not-generated-tolerance-5/5889/5

Will ask for

Suggestions on how to fix it

Initialise maxid to 1 (third line of the code snippet above)

Version

From user info: HADDOCK3 Version: v2025.9.0

[amjjbonvin]

Asked the user for all his input data and script as I can not reproduce the problem.