/usr/share/gromacs/top/charmm27.ff/dna.n.tdb is in gromacs-data 2018.1-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 | [ None ]
[ 5' ]
[ delete ]
P
O1P
O2P
[ replace ]
O5' O5' ON5 15.9994 -0.66
C5' C5' CN8B 12.011 0.05
[ Add ]
1 2 H5T O5' C5' C4'
HN5 1.008 0.43 -1
|