psi3: 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/psi3 - from package: psi3

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

Error in other languages:
psi3: bevel nie gevind nie
psi3: comandă negăsită
psi3: comando não encontrado
psi3: commande introuvable
psi3: command not found
psi3: käsku ei ole
psi3: không tìm thấy lệnh
psi3: komanda nerasta
psi3: Kommando nicht gefunden.
psi3: kommandot finns inte
psi3: komut yok
psi3: nie znaleziono polecenia
psi3: níor aimsíodh an t-ordú
psi3: no se encontró la orden
psi3: no s'ha trobat l'ordre
psi3: opdracht niet gevonden
psi3: parancs nem található
psi3: perintah tidak ditemukan
psi3: príkaz nenájdený
psi3: příkaz nenalezen
psi3: команда не найдена
psi3: командата не е открита
psi3: 命令找不到
psi3: コマンドが見つかりません