While the molecular dynamics for weakly coupled, harmonic oscillators undergoing infinitesimal amplitude displacements are well described by normal modes, and the other extreme — strongly coupled anharmonic, large amplitude oscillating units can be treated as though they are ergodic systems, so that only state densities are needed, intermediate cases are very difficult to analyze. Here we propose that some intermediate cases, wherein we seek relative rates for reaction induced by specific frequency excitation (three-center displacement reactions or dissociation), may be approximately estimated from a normal mode analysis by calculating increment in root mean square amplitudes for a suitable internal coordinate, upon specific multiple photon absorption. A test case is presented as an illustration.