mpirun.ch_gen2: 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/mvapich/bin/mpirun.ch_gen2 - from package: mvapich
/usr/lib/mvapich/bin/mpirun.ch_gen2 - from package: mvapich

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

Error in other languages:
mpirun.ch_gen2: bevel nie gevind nie
mpirun.ch_gen2: comandă negăsită
mpirun.ch_gen2: comando não encontrado
mpirun.ch_gen2: commande introuvable
mpirun.ch_gen2: command not found
mpirun.ch_gen2: käsku ei ole
mpirun.ch_gen2: không tìm thấy lệnh
mpirun.ch_gen2: komanda nerasta
mpirun.ch_gen2: Kommando nicht gefunden.
mpirun.ch_gen2: kommandot finns inte
mpirun.ch_gen2: komut yok
mpirun.ch_gen2: nie znaleziono polecenia
mpirun.ch_gen2: níor aimsíodh an t-ordú
mpirun.ch_gen2: no se encontró la orden
mpirun.ch_gen2: no s'ha trobat l'ordre
mpirun.ch_gen2: opdracht niet gevonden
mpirun.ch_gen2: parancs nem található
mpirun.ch_gen2: perintah tidak ditemukan
mpirun.ch_gen2: príkaz nenájdený
mpirun.ch_gen2: příkaz nenalezen
mpirun.ch_gen2: команда не найдена
mpirun.ch_gen2: командата не е открита
mpirun.ch_gen2: 命令找不到
mpirun.ch_gen2: コマンドが見つかりません