Fatal error:
Group Protein_UNK referenced in the .mdp file was not found in the index file.
Group names must match either [moleculetype] names or custom index group
names, in which case you must supply an index file to the '-n' option
of grompp.
通过查找发现有同学在命令后加-n index.ndx能解决问题，于是尝试了一下，后报以下错误：
Error in user input:
Invalid command-line options
In command-line option -n
Option specified multiple times