In the author's last paper [Ikeda, Appl. Phys. Express 13, 015508 (2020)], it was suggested that the number of molecules that constitute a critical nucleus of pentacene is about ten based on the results of molecular dynamics (MD) simulations performed to investigate the stability of clusters comprising standing pentacene molecules on substrate surfaces. In this study, the author carried out additional MD simulations of clusters comprising ten pentacene molecules and found that these molecular clusters of critical size show stochastic behavior, which can be understood based on curves of free energy change. The discrepancy in the number of molecules that constitute a critical nucleus of pentacene between previous reports and this study is also discussed.