  A diabatic (configuration-fixed) constrained approach to calculate the
potential energy surface (PES) of the nucleus is developed in the relativistic
mean field model. {As an example}, the potential energy surfaces of $^{208}$Pb
obtained from both adiabatic and diabatic constrained approaches are
investigated and compared. {It is shown that} the diabatic constrained approach
enables one to decompose the segmented PES obtained in usual adiabatic
approaches into separate parts uniquely characterized by different
configurations, {to follow the evolution of single-particle orbits till very
deformed region}, and to obtain several well defined deformed excited states
which can hardly be expected from the adiabatic PES's.
