We consider the effective interaction between dendrimers affected by pH and counterions of solution as  well as terminal modification of dendrimers through the calculation of the free energy with coarsegrained  molecular dynamics simulations. We find that the decrease of the pH value can induce the
increase in the effective size of dendrimers and that multivalent counterions can lead to like-charged  attraction between dendrimers. Under high-valent counterion conditions, the attractive force between  the two dendrimers displays the ‘‘M’’-like profiles induced by the competition of various interactions.  Moreover, short modification of surface does not change their trend of aggregation no matter what the  terminal monomers of the dendrimers are. Our results are helpful for designing a novel class of  dendrimers to achieve optimal functionalities.