ii (ω) is identified as a probability distribution. With m = 2 this method is equivalent to a normal modes analysis performed with the thermally averaged Hessian matrix . Once a set of internal coordinates ζ is defined to initialize the optimization procedure, calculating the stationary variations of Ω (m) with respect to the Z -1 ik yields the optimal linear combination of ζ that produces a set of effective modes q with maximally localized absorption bands. Note that P (q)