Is it possible to define the coordinates of an atom using other coordinates while optimizing in gaussian 16? I know it is possible to fix the value of composite expressions using generalized internal coordinates (gic) in gaussian 16. But is it possible to assign that value to the coordinate of another atom.

For example if I have the following coordinate file 

H x1 y1 z1
H x2 y2 z2
O x3 y3 z3

Is it possible to assign x3 = (x1 + x2)/2 . When optimizing this structure I would like that the x coordinate of oxygen always be the average of the x coordinates of the two hydrogen atoms.
I just figured out that there is a way to do this. Though its not that clean I think. I can define the initial structure where x3 = (x1+x2)/2 and then freeze the following expression using gic.

x3 - (x1+x2)/2