[gmx-users] mean square displacement of the chain center of mass

[ARGYRIOS KARATRANTOS]

i have some polymeric chain sin a water solvent. and i want to measure the mean
square displacement of the chain center of mass during time t, 

what option in the g_msd command should i select for the msd of the center of
mass of chain

thanks in advance
Argyris Karatrantos