We consider a time-dependent Schrödinger equation whose Hamiltonian is a $2\times 2$ real symmetric matrix. We study, using an exact WKB method, the adiabatic limit of the transition probability in the case where several complex eigenvalue crossing points accumulate to one real point.
"Adiabatic transition probability for a tangential crossing." Hiroshima Math. J. 36 (3) 443 - 468, November 2006. https://doi.org/10.32917/hmj/1171377083