It is planned to include adaptive optics as an integral part of the 10-m Gran Telescopio Canarias. Preliminary scientific requirements have been drawn up, and it is necessary to identify systems which will satisfy these requirements. The approach adopted here is to use the LACE program to model the adaptive-optics system. The program provides for the use of turbulence profiles together with a detailed description of the adaptive system to predict the performance. This has been used to determine performance as measured by the Strehl ratio, as a function of a number of parameters, including guide-star magnitude and offset. This information, combined with a model of the angular density of Galactic stars on the sky, has allowed estimation of sky coverage. The sky coverage has been estimated for high-order and low-order systems using natural or laser guide stars.