pwi2xsf.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/pwi2xsf.x_mpich - from package: quantum-espresso-mpich

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

Error in other languages:
pwi2xsf.x_mpich: bevel nie gevind nie
pwi2xsf.x_mpich: comandă negăsită
pwi2xsf.x_mpich: comando não encontrado
pwi2xsf.x_mpich: commande introuvable
pwi2xsf.x_mpich: command not found
pwi2xsf.x_mpich: käsku ei ole
pwi2xsf.x_mpich: không tìm thấy lệnh
pwi2xsf.x_mpich: komanda nerasta
pwi2xsf.x_mpich: Kommando nicht gefunden.
pwi2xsf.x_mpich: kommandot finns inte
pwi2xsf.x_mpich: komut yok
pwi2xsf.x_mpich: nie znaleziono polecenia
pwi2xsf.x_mpich: níor aimsíodh an t-ordú
pwi2xsf.x_mpich: no se encontró la orden
pwi2xsf.x_mpich: no s'ha trobat l'ordre
pwi2xsf.x_mpich: opdracht niet gevonden
pwi2xsf.x_mpich: parancs nem található
pwi2xsf.x_mpich: perintah tidak ditemukan
pwi2xsf.x_mpich: príkaz nenájdený
pwi2xsf.x_mpich: příkaz nenalezen
pwi2xsf.x_mpich: команда не найдена
pwi2xsf.x_mpich: командата не е открита
pwi2xsf.x_mpich: 命令找不到
pwi2xsf.x_mpich: コマンドが見つかりません