Universal variational expansion for three-body systems

A universal variational expansion in the interparticle coordinates is proposed for the accurate calculation of the bound states of an arbitrary three-body system. This fully non-adiabatic variational expansion has been applied successfully to various three-body systems with different masses, including the so-called nearly adiabatic systems (two particle masses are significantly greater than the third mass). Accurate calculations for the bound states in the latter adiabatic systems present a very complicated problem for the usual variational approaches in relative coordinates.