nwchem_openmpi: 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/openmpi/bin/nwchem_openmpi - from package: nwchem-openmpi

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

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