I am trying to fitting the bond probability distribution of coarse-grained model of polyethylene(PE) with small polymeric system for understanding the process of coarsed-graining and IBI method. The mapping scheme of coarsed-graining for PE is 1:1,(i.e., Repeat Unit is CH2). However, when the modify input data for coarsed-graining which is merely different from the data for all atom simulation in the bond,angle and dihedral potential, and the CG data was performed to execute the first iteration simulation, I found the big difference between all atom and coarse-graining model in the trajectory. Could anyone tell me whether I ignore certain details when I modify the details? Furthermore, for the testify of my confusion, the 2nd iteration was performed. Compared to result of the 1st iteration, the probability of bond distribution is slightly higher. Do you have any helpful information to share with me? Appreciate any comments. Tons of thanks.