[gmx-users] gmx velacc

[dgfd dgdfg]

Hi. This is my first letter to this list, sorry.
version:  :-) GROMACS - gmx help, 2016.4 (-:
Problem with velacc command ( http://manual.gromacs.org/documentation/2016.4/onlinehelp/gmx-velacc.html  ).
In the folder with calculated long trajectory I input:

gmx velacc -f traj.trr -mol -b 10 -e 20

and select necessary group after question

GROMACS output:

Split group of 147 atoms into 1 molecules
trr version: GMX_trn_file (single precision)
Last frame         -1 time    0.000   
Last frame         -1 time 1100.000   
Not enough frames in trajectory - no output generated.


What is wrong in my input and why "Last frame" is printed twice?