I see, thank you.
Do you think that these pull code options in my md.mdp would suffice? The Mg ion is a_10900 and the ligand oxygen is a_10908.
; Pull code
pull = yes
pull-ngroups = 2
pull-ncoords = 1
pull-group1-name = a_10908
pull-group2-name = a_10900
pull-coord1-type = umbrella
pull-coord1-geometry = distance ; simple distance increase
pull-coord1-groups = 1 2
pull-coord1-dim = Y Y Y
pull-coord1-rate = 0.0 ; not pull, just distance restraint at a reference
distance bw 2 groups
pull-coord1-k = 1000 ; kJ mol^-1 nm^-2
pull-coord1-start = yes ; define initial COM distance > 0
pull-nstxout = 5000
pull-nstfout = 5000