A portion of a layout of a single layer of an integrated circuit is to be multi-patterned. The patterns are divided into first and second groups, to be patterned on the single layer by a first mask or a second mask. For each portion of each pattern, a spacing relationship is determined between that portion and any adjacent pattern on either or both sides. A processor computes a first capacitance (C), resistance (R), or resistance-capacitance (RC) cost of assigning the first group to the first mask and the second group to the second mask, and a second cost of assigning the first group to the second mask and the second group to the first mask, based on the spacing relationships. The first group is assigned to the first mask and the second group to the second mask if the first cost is lower than the second cost.