g_demux.pl: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/g_demux.pl - from package: gromacs

You may install the required package with command yum install {package}
Error in other languages:
g_demux.pl: bevel nie gevind nie
g_demux.pl: comandă negăsită
g_demux.pl: comando não encontrado
g_demux.pl: commande introuvable
g_demux.pl: command not found
g_demux.pl: käsku ei ole
g_demux.pl: không tìm thấy lệnh
g_demux.pl: komanda nerasta
g_demux.pl: Kommando nicht gefunden.
g_demux.pl: kommandot finns inte
g_demux.pl: komut yok
g_demux.pl: nie znaleziono polecenia
g_demux.pl: níor aimsíodh an t-ordú
g_demux.pl: no se encontró la orden
g_demux.pl: no s'ha trobat l'ordre
g_demux.pl: opdracht niet gevonden
g_demux.pl: parancs nem található
g_demux.pl: perintah tidak ditemukan
g_demux.pl: príkaz nenájdený
g_demux.pl: příkaz nenalezen
g_demux.pl: команда не найдена
g_demux.pl: командата не е открита
g_demux.pl: 命令找不到
g_demux.pl: コマンドが見つかりません