<div>I am running a comparison between AMBER and Gromacs for a AOT system. First I created the AMBER force field and then transformed it to Gromacs force field. I did 1 step of MD respectively in AMBER and Gromacs with the same configuration. All the energy terms including bond, angle, dihedral, LJ (SR), LJ (14) and Coulomb (SR) are the same with each other between the two runs except the Coulomb (14) interaction. What might be the reason for this? I have totally no idea. Because the LJ (14) and Coulomb (14) calculation use the same 1-4 pairs in Gromacs, the LJ (14) is the same which means the 1-4 pairs are the same between AMBER and Gromacs. The Coulomb (SR) is the same which means the charges are the same between AMBER and Gromacs. (I also checked these two things explicitly between AMBER and Gromacs.) How could the Coulomb (14) be different then? Does any one have any idea about how the coulomb (14) interactions are calculated in AMBER and Gromacs? Thanks a lot.