mdrun_mpi_d.openmpi: 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_mpi_d.openmpi - from package: gromacs-openmpi

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

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