Four types of solvers are provided in MODFLOW: the geometric multi-grid (GMG), the strongly implicit procedure (SIP), the preconditioned conjugate gradient method (PCG2), and the slice-successive overrelaxation method (SSOR). One of these four methods must be selected in the Packages dialog accessed through the Global Options\Basic Package dialog.
Once a solver has been selected, the appropriate solver package dialog can be accessed through one of the solver commands (LMG, SIP, PCG2, or SSOR) in the MODFLOW menu. The default values shown in each dialog are typically adequate.
Related Links:
Packages Supported by GMS
Building a MODFLOW Model