g_mdrun_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 CentOS possible default full paths are:

/usr/lib64/openmpi/bin/g_mdrun_openmpi - from package: gromacs-openmpi

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

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