In this expression, we have omitted the second term in Eq. ( ), i.e., the self-polarization term, which is usually negligible when considering a single-mode cavity. We confirmed this by including this term in our numerical calculations, and indeed its presence did not contribute significantly to the results. In Eq. ( ), our model is defined such that the diagonal elements of dipole operator in the diabatic basis are zero and the off-diagonal elements, i.e., the transition dipole moment, is a constant function of nuclear geometries of value µ 12 = 1.0 ea 0 (Condon approximation). Furthermore, we choose Z = 1.0 e.