The default is:
simulate(modelName, method="dassl")

You can also try some alternative solvers:
"euler" is a fixed-step, explicit euler solver.
"rungekutta" is supposedly fourth-order.
"dassl2" uses the same event system as the euler solver, and the same solver as "dassl". I haven't checked it in-depth, so I'm not sure what it does differently from "dassl".

thank you very much