g_principal: command not found

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

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

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_principal - from package: gromacs

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/g_principal - from package: gromacs

You may install the required package with command apt-get install {package}

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