Abstract
The symmetric division deg (SDD) index is one the 148 discrete Adriatic indices, introduced several years ago. The SDD index has already been proved a valuable index in the QSPR/QSAR (quantitative structure-property/activity relationships) studies. In the present paper, we firstly correct an upper bound on the SDD index of molecular trees, reported in the recent paper [MATCH Commun. Math. Comput. Chem. 82 (2019) 43-55], by giving the best possible upper bound on the SDD index of any molecular (n, m)-graph (a molecular graph with order n and size m). We then establish a lower bound on the SDD index of any molecular (n,m)-graph. Finally, by extending a theorem of the aforementioned paper, we characterize the graphs with fifth to ninth minimum SDD indices from the class of all molecular trees having a fixed, but sufficiently large, order.