1. A method for circuit design, comprising: identifying a parametric cell (P-cell) in a design of an integrated circuit, the P-cell comprising a closed environment containing a transmission line (T-line) and one or more elements of the design selected from a group of the elements consisting of crossing lines and parallel lines in a neighborhood of the T-line; optimizing a geometry of the P-cell in order to minimize a sensitivity of the P-cell to a design environment of the P-cell; providing a parametric model of the P-cell, the parametric model defining electrical properties of the P-cell as a function of geometrical parameters of the P-cell; and simulating, on a computer, the design of the integrated circuit using the parametric model wherein simulating the design comprises identifying a critical path in the design, formulating design requirements with respect to the critical path, choosing the P-cell based on the design requirements, and performing small-circuit simulations to determine whether the design requirements are met.