/usr/share/gromacs/top/gmx.ff/ is in gromacs-data 4.5.5-2.
This file is owned by root:root, with mode 0o755.
..
/usr/share/gromacs/top/gmx.ff/1mlg.itp
/usr/share/gromacs/top/gmx.ff/2mlg.itp
/usr/share/gromacs/top/gmx.ff/aminoacids.c.tdb
/usr/share/gromacs/top/gmx.ff/aminoacids.hdb
/usr/share/gromacs/top/gmx.ff/aminoacids.n.tdb
/usr/share/gromacs/top/gmx.ff/aminoacids.rtp
/usr/share/gromacs/top/gmx.ff/aminoacids.vsd
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/usr/share/gromacs/top/gmx.ff/watermodels.dat