The N-terminal region in the capsid protein of many viruses such as brome mosaic virus, cowpea chlorotic mottle virus, southern bean mosaic virus and Sesbania mosaic virus is believed to function as a molecular switch in controlling the particle polymorphism. When present, it stabilizes the six-fold axes that are common in T=3 capsids; deletion of the N-terminal arm in T=3 capsids results in the formation of T=1 capsids. Inspired by this finding, we have set out to perform structural studies on the assembly and structure of capsids that are formed by deletion and substitution mutants of the coat protein. We will employ a newly developed united-atom model that is based on the coordinates from the already solved crystal structures that are available on the VIPER database. This model is used in conjunction with the rigid-body Monte Carlo simulation method and the discontinuous molecular dynamics algorithm - a fast alternative to standard molecular dynamics. Our studies can provide information on the key residues that are required for correct assembly of the Sesbania mosaic virus and alternative assembly of T=1 capsids.