[gmx-users] charge density of multiple frames in a limited region

[p buscemi]

Dear Users,
In using:
gmx density -f lipid.nvt.trr -s octanoate.nvt3.tpr -dens charge -center -symm -sl 100 -b 4 -e 4.5
it appears that the average charge density of all frames and the total box size is used in the calculation. Is there a way to
1) specify multiple ranges to output say, every , 20th frame other than running the command multiple times and
2) specify at least one of the x and y dimensions in which the charge density is calculated ?

Regards
pb