adapt group-ID with time

Dear all,

Is a the group definition of atom defined for instance like :
> variable ismoving atom vx>0.01
> group moving variable ismoving
is evolving each time step (and with atom velocity) ?
It seems to me that it is only computed once at the beginning...

I would like to "dump" only certains atoms, which velocities are greater than 0.01, to save space.
How could I do that without a group definition ?

Thks !

I managed to find this useful command :

dump_modify dmp1 thresh vx > 0.01

that's the way to do it - not via changing the group definition