Hello all , I want to create the forcefield for methyl arsonous acid , as parameters for arsenic is not there in any of the Amber , CHARM or gromacs .I am new to molecular dynamics .I looked for MCPB.py module in Amber tools but I am not able to understand the steps ,how to do it . Can any one help me to find the forcefield for the same using MCPB.py or if there is any other way to find the same .Kindly help !!! Thank You