/usr/share/gromacs/top/gromos54a7.ff/aminoacids.r2b 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 | ; rtp residue to rtp building block table
;GMX Force-field
CYS CYSH
HISD HISA
HISE HISB
LYS LYSH
LYSN LYS
HEM HEME
|