The effective force for Schwarzschild coordinates is F = − − G M m u 2 ( 1 + 3 ( h u c ) 2 ) = − − G M m r 2 ( 1 + 3 ( h r c ) 2 ) . where the second term is an inverse-quartic force corresponding to quadrupole effects such as the angular shift of periapsis (It can be also obtained via retarded potentials).