g_saxs: 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 Debian possible default full paths are:

/usr/bin/g_saxs - from package: gromacs

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

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