g_mdmat: 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_mdmat - from package: gromacs

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

On Debian possible default full paths are:

/usr/bin/g_mdmat - from package: gromacs

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

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