Abstract: Approximations for previewer-based decomposition are disclosed. In one embodiment, the previewer uses only resistors, capacitors, and controlled sources. Thus, at least some circuit elements that are not resistors or capacitors may be replaced with some combination of resistors, capacitors, or controlled sources. For example, inductors can be modeled as short circuits or as a non-zero resistance. As another example, transistors can be replaced by some combination of resistors, capacitors, and controlled sources. In one embodiment, a diagonal approximation is used to form an approximation for simulation in a previewer-based decomposition. The diagonal approximation uses resistors, capacitors, or controlled sources, in one embodiment. In one embodiment, an impedance between two partitions is split to form a new previewer node.