psi4: 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 Debian possible default full paths are:

/usr/bin/psi4 - from package: psi4

You may install the required package with command apt-get install {package}

Error in other languages:
psi4: bevel nie gevind nie
psi4: comandă negăsită
psi4: comando não encontrado
psi4: commande introuvable
psi4: command not found
psi4: käsku ei ole
psi4: không tìm thấy lệnh
psi4: komanda nerasta
psi4: Kommando nicht gefunden.
psi4: kommandot finns inte
psi4: komut yok
psi4: nie znaleziono polecenia
psi4: níor aimsíodh an t-ordú
psi4: no se encontró la orden
psi4: no s'ha trobat l'ordre
psi4: opdracht niet gevonden
psi4: parancs nem található
psi4: perintah tidak ditemukan
psi4: príkaz nenájdený
psi4: příkaz nenalezen
psi4: команда не найдена
psi4: командата не е открита
psi4: 命令找不到
psi4: コマンドが見つかりません