Thanks for your further comments on the command 'COOR DYNA'.
I had been briefed about the "average" feature of 'COOR DYNA' before I started trying this workaround. Since I only deal with a single frame at a time, I think it should be fine. I tested both 'COOR DYNA' and 'TRAJ READ' locally and compared the "INTEraction" outputs, which turned out to be the same. But please kindly correct me if I am wrong.
Also, I am glad to learn the faster way, i.e., "COOR READ file file ... CONT". I appreciate it.