lammps consultation

Dear Professor:
Merry Christmas!
My name is cheng yang, and I am a graduate student at University of Science and Technology Beijing in China.Excuse me for this abrupt letter.I would like to ask some questions about the lammps. I am using MD (LAMMPS software) to find out that whether the diamond tip’s vibration have some effect on friction and the substrate is graphite. After relaxation ,we displaced a harmonic spring laterally coupled to the we connected the tip and the virtual atom through the a harmonic spring . Then the virtual atom move at a linear velocity. But I want to make the Z coordinate of the virtual atom’ s is same as tip‘s center of mass. And the order is “set atom 1802 Z V_pgtipz". The atom 1802 is the virtual atom and the v_pgtipz is the tip‘s center of mass in the z direction .But it can not realize ,that is to say,the coordinate of the virtual atom in the Z direction does not change ,so the order is invalid . I want to know the reason and whether there is another method to ralize it .

Thank you, and I look forward to hearing from you.

Best regards,
Cheng Yang

School of Mechanical Engineering
University of Science and Technology Beijing
Beijing, China

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