/usr/share/gromacs/top/charmm27.ff/aminoacids.r2b is in gromacs-data 4.5.5-2.
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 | ; rtp residue to rtp building block table
;GMX Force-field
HISD HSD
HISE HSE
HISH HSP
LYSN LSN
ASPH ASPP
GLUH GLUP
HEM HEME
|