We present a new formulation for the dynamicmodeling of twisted nematic liquid crystal display cells based on aconstant charge assumption instead of the commonly used "fixedvoltage" model. This formulation is based on a tensor representationof the director field, which is solved using a combination offinite-element and finite-difference methods. It allows the accuratemodeling of liquid crystal pixel cells in thin-film-transistor-drivenactive matrix displays, extensively used in flat panel computer andtelevision displays.