Hi, there: It seems to me that there was a bug at line 214 of function read_next_vmd_frame() in file ./src/gmxlib/vmdio.c. vec[0] = .1*ts.A; vec[1] = .1*ts.B; vec[2] = .1*ts.B; should be changed to : vec[0] = .1*ts.A; vec[1] = .1*ts.B; vec[2] = .1*ts.C; This is identified in gromacs4.5.1 version. Thanks, Bin