  The infrared response of a system of two vibrational modes in a cavity is
calculated by an effective non-Hermitian Hamiltonian derived by employing the
nonequilibrium Green's functions (NEGF) formalism. Degeneracies of the
Hamiltonian (exceptional points, EP) widely employed in theoretical analysis of
optical cavity spectroscopies are used in an approximate treatment and compared
with the full NEGF. Qualitative limitations of the EP treatment are explained
by examining the approximations employed in the calculation.
