[gmx-users] Inconsistent between velocity and force data?

xzp02 at mails.tsinghua.edu.cn xzp02 at mails.tsinghua.edu.cn
Thu Aug 25 09:43:19 CEST 2005

Dear all:
        I am running simulations on a system with several residues, c. o. m. data
of one residue is attracted by g_traj -com, two approaches were made to get total
force on the residue.

1) g_traj -of xxx -com
force data f_g is obtained in unit kJ/mol/nm, and the force is calculated by 
f1 = f_g * 1/NA * (N_residue/N_tot), where NA is the Avogadro number, N_residue
and N_tot is the number of atoms of the residue and whole system seperatly.
2) g_traj -ov yyy -com
velocity data v_g is obtained in unit nm/ps, and the force is calculated by
f 2= m * d(v_g)/dt, where m is total mass of the residue and d means

unexpectly I found the value f1 and f2 differs by a factor near 1000!, I am not
sure about the (1/mol) definition of the unit in this case. Can anyone give me
some advices?

    Thanks a lot.


Xu Zhiping 
Ph.D Candicate
Department of Engineering Mechanics
Tsinghua University
Beijing, 100084, CHINA

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