g_velacc: 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_velacc - from package: gromacs

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

On Debian possible default full paths are:

/usr/bin/g_velacc - from package: gromacs

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

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