1. A method executed in a computer system for producing a combined system of partial differential equations comprising: representing each of a plurality of systems as an application mode modeling physical quantities of said each system; determining a representation of a partial differential equation system for each application mode corresponding to one of said plurality of systems using at least one non-local coupling, said at least one non-local coupling defining a value from a first portion of a first domain to another part of a second domain; and forming said combined system of partial differential equations using partial differential equation systems associated with said plurality of systems.