Hi everyone
I want to get the center of mass velocity autocorrelation function,so I define a center for a molecule and the put the center in a set,finally I use the analyse tools in forcite to analyse the  velocity autocorrelation function of the center(the set),but there is an error message "There is no moved particles for analyse",however the molecule is moving because I give it a velocity.I konw the VAC is used for atoms,but is there any ways to get the center of mass velocity autocorrelation function even by script?The link is paper about the the center of mass velocity autocorrelation function.http://scitation.aip.org/content/aip/journal/jcp/131/22/10.1063/1.3271829

Thanks in advance 
Sincerely
Li