Mark center

Mark_center is a small callback that puts a cross at the center of space, where the camera is pointing.

You could also use this to setup arbitrary CGOs (points, lines, etc) in 3D space.

from pymol import cmd

def crosshair_put_center():
   t = cmd.get_position()
   m = [1, 0, 0, t[0], 0, 1, 0, t[1], 0, 0, 1, t[2], 0, 0, 0, 1]
   cmd.set_object_ttt('crosshair', m, homogenous=1)

cmd.load_callback(crosshair_put_center, '_crosshair_cb')

cmd.pseudoatom('crosshair', pos=(0,0,0))
cmd.show_as('nonbonded', 'crosshair')