g_helixorient: 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/bin/g_helixorient - from package: gromacs

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

On Debian possible default full paths are:

/usr/bin/g_helixorient - from package: gromacs

You may install the required package with command apt-get install {package}

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