mpich2version: 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/mpich2/bin/mpich2version - from package: mpich2
/usr/lib/mpich2/bin/mpich2version - from package: mpich2

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

Error in other languages:
mpich2version: bevel nie gevind nie
mpich2version: comandă negăsită
mpich2version: comando não encontrado
mpich2version: commande introuvable
mpich2version: command not found
mpich2version: käsku ei ole
mpich2version: không tìm thấy lệnh
mpich2version: komanda nerasta
mpich2version: Kommando nicht gefunden.
mpich2version: kommandot finns inte
mpich2version: komut yok
mpich2version: nie znaleziono polecenia
mpich2version: níor aimsíodh an t-ordú
mpich2version: no se encontró la orden
mpich2version: no s'ha trobat l'ordre
mpich2version: opdracht niet gevonden
mpich2version: parancs nem található
mpich2version: perintah tidak ditemukan
mpich2version: príkaz nenájdený
mpich2version: příkaz nenalezen
mpich2version: команда не найдена
mpich2version: командата не е открита
mpich2version: 命令找不到
mpich2version: コマンドが見つかりません