neb.x_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/neb.x_openmpi - from package: quantum-espresso-openmpi

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

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