Lanthanide (Ln) systems have been widely applied for optical materials because they have bright emissions originating from f–f transitions. To design better optical materials, the information of emission and quenching mechanism is indispensable. However, it is too demanding to apply ab initio calculations for Ln systems due to their strong electron correlation and spin-orbit interaction. To overcome this problem, I proposed a new approximation, the energy shift method, which enabled us to perform the exhaustive exploration of critical points, such as local minima, transition states, and crossing points, in all internal degrees of freedom on and between potential energy surfaces of large Ln systems. This review briefly outlines the basic concepts of the Ln luminescence, followed by the overview of the energy shift method and its application studies for thermo-sensors and strong emitters.