gpaw-mpisim: command not found

Have You tried accessing the binary with the full path?

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/gpaw-mpisim - from package: gpaw

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