The diabatic couplings are the off-diagonal terms of wave functions written in a diabatic basis. There is a need to use the diabatic basis when there is a breakdown of the Born-Oppenheimer approximation. Then a transformation is done from the adiabatic to diabatic basis. But what does non-adiabatic coupling mean?
The reason I am asking is that most quantum chemistry softwares (I use Molpro) calculate the non-adiabatic coupling. Then some procedure has to be used to convert them to diabatic coupling. But what is the difference between the two and why do I need to use the diabatic coupling?