  We model the spontaneous assembly of a capsid (a virus's closed outer shell)
from many copies of identical units, using entirely irreversible steps and only
information local to the growing edge. Our model is formulated in terms of (i)
an elastic Hamiltonian with stretching and bending stiffness and a spontaneous
curvature, and (ii) a set of rate constants for addition of new units or bonds.
An ensemble of highly irregular capsids is generated, unlike the well-known
icosahedrally symmetric viruses, but (we argue) plausible as a way to model the
irregular capsids of retroviruses such as HIV. We found that (i) the
probability of successful capsid completion decays exponentially with capsid
size; (ii) capsid size depends strongly on spontaneous curvature and weakly on
the ratio of the bending and stretching elastic stiffnesses of the shell; (iii)
the degree of localization of Gaussian curvature (a measure of facetedness)
depends heavily on the ratio of elastic stiffnesses.
