ph.x_mpich: 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/mpich/bin/ph.x_mpich - from package: quantum-espresso-mpich

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

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