Valiev and co-workers noted that the density of states can be fitted and approximated as per Figure . Here, the criteria for the strongest transition participating mode is probed, and estimated within one of three regimes as a function of E mn . Either the strongest mode has an energy larger than 1000 cm -1 and a Huang-Rhys factor between 0.1 and 0.5, has an energy smaller than 1000 cm -1 and a Huang-Rhys factor greater than 0.1 and 0.5, or has a Huang-Rhys factor larger than 0.5. The former regime was used by Plotnikov & Dolgikh .