head.x: 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/bin/head.x - from package: quantum-espresso

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

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