Diabatic state Quote:Diabatic process: Rapidly changing conditions prevent the system from adapting its configuration during the process, hence the probability density remains unchanged. Typically there is no eigenstate of the final Hamiltonian with the same functional form as the initial state. The system ends in a linear combination of states that sum to reproduce the initial probability density.
Ref:
http://en.wikipedia.org/wiki/Adiabatic_theorem#Diabatic_vs._adiabatic_processes The Hamiltonian matrix is diagonal in the adiabatic basis. In the diabatic basis it is not diagonal. The off-diagonal hamilton matrix elements couple the diabatic states. Combinations of the diabatic wavefunctions make the hamiltonian diagonal.