turbo_davidson.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/turbo_davidson.x - from package: quantum-espresso

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

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