collisions.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/lib64/mpich/bin/collisions.x - from package: ga-mpich
/usr/lib/mpich/bin/collisions.x - from package: ga-mpich
/usr/lib64/openmpi/bin/collisions.x - from package: ga-openmpi

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

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