nwchem_mpich: 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/nwchem_mpich - from package: nwchem-mpich

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

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