1. A method of simulating electromagnetic characteristics of an electrical circuit, comprising the steps of: receiving netlist data defining component instances and their topological interconnection in an electrical circuit, said data including layout component instances; and performing a circuit simulation using said netlist data, and causing said circuit simulation to use a model for each said layout component instance by: attempting to retrieve an existing simulation model of said layout component instance from a database of such layout component simulation models; if no suitable simulation model can be found in the database, attempting to interpolate a new simulation model from among existing simulation models in the database; and if interpolation is determined not to be feasible, performing an electromagnetic simulation of said layout component instance to develop a new electromagnetic simulation model, using said new model in the circuit simulation, and adding said new model to the database for future use.