mdrun_d: 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/mdrun_d - from package: gromacs

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

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