There are various macros to Get coordinates of atoms for use in other equations. Different ones are needed for regular sites and for point_for_sites in rigid bodies. Note that you can't use information from point_for_sites of a rigid body in regular sites.

For regular sites use:

{ Get(Find_Child(Get(site_recs), site_name), x) }
{ Get(Find_Child(Get(site_recs), site_name), y) }
{ Get(Find_Child(Get(site_recs), site_name), z) }

If it's more convenient use the macro:

macro Get_x(site_name) { Get(Find_Child(Get(site_recs), site_name), x) }
macro Get_y(site_name) { Get(Find_Child(Get(site_recs), site_name), y) }
macro Get_z(site_name) { Get(Find_Child(Get(site_recs), site_name), z) }

For rigid body point for sites you can get the coordinates in cartesian space with the macro:

macro & Get_x(site_name) { Point(site_name, rx) }        
macro & Get_y(site_name) { Point(site_name, ry) }        
macro & Get_z(site_name) { Point(site_name, rz) }       

In the future it might be possible to work with the fractional coordinates of other atoms in the rigid body by switching from rx to ua coordinates with topas.inc macros:

Point_ua(site_name)
Point_ub(site_name)
Point_uc(site_name)