From XMS Wiki
solver packages for MODFLOW supported by GMS. The GMG solver is an iterative solver based on a multi-grid approach.The GMG package is one of the
- Inner convergence residual (RCLOSE) – The algorithm computes the 12-norm of the residual and compares it against RCLOSE.
- Maximum inner iterations (IITER) – For each linear solution. The default is 100. Specify a smaller number for nonlinear problems to prevent an excessive number of inner iterations.
- Outer convergence residual (HCLOSE) – For nonlinear problems. After each linear solve (inner iteration), the maximum norm of the head change is compared against HCLOSE. Can be set to a large number for linear problems. Ignored if Maximum outer iterations is 1.
- Maximum outer iterations (MAXITER) – Also referred to as MXITER. For linear problems, set to 1. For nonlinear problems, set to a higher number though it is usually unnecessary to go above 100.
- Damping parameter (DAMP) – A value of 1.0 should be used for linear problems. For nonlinear problems, a value less than 1.0 but greater than 0.0 may be necessary to achieve convergence.
- Damping option (IADAMP) – Flag that controls adaptive damping. Contains the following options:
- "(0) Constant"
- "(1) Cooley adaptive"
- "(2) RRR adaptive"
- Maximum damping (DUP)
- Minimum damping (DLOW)
- Maximum head change (CHGLIMIT)
- Output flag (IOUTGMG) – Flag that controls the output of the GMG solver. Values can be 0–4.
- 0 = Only the solver inputs are printed.
- 1 = For each linear solve, prints the number of PCG iterations, the value of the damping parameter, the 12-norm of the residual, and the maximum norm of the head change and its location (column, row, layer). At the end of a time/stress period, the total number of GMG calls, PCG iterations, and a running total of PCG iterations for all time/stress periods are printed.
- 2 = The convergence history of the PCG iteration is printed, showing the 12-norm of the residual and the convergence factor for each iteration.
- 3 = Same as 1 except the output is sent to the terminal instead of the output file.
- 4 = Same as 2 except the output is sent to the terminal instead of the output file.
- Maximum head output (IUNITMHC)
- Multi-grid preconditioner smoothing (ISM) – Flag that controls the type of smoother used in the multi-grid preconditioner.
- "(0) ILU smoothing" – This smoothing requires an additional vector on each multigrid level to store the pivots in the ILU factorization.
- "(1) SGS smoothing" – Symmetric GaussSeidel (SGS) smoothing.
- Multi-grid preconditioner coarsening (ISC) – Flag that controls semi-coarsening in the multi-grid preconditioner.
- "(0) Row, Col., Layer" – Coarsens all the rows, columns and layers.
- "(1) Row, Col." – Coarsens all the rows and columns.
- "(2) Col., Layer" – Coarsens all columns and layers.
- "(3) Row, Layer" – Coarsens all rows and layers.
- "(4) None" – No coarsening.
- Relocation parameter (RELAX) – Used to improve the spectral condition number of the ILU preconditioned system.
For more info on the GMG solver look at the following document:
- Wilson, John D., and Richard L. Naff. MODFLOW-2000, The US Geological Survey Modular Ground-Water Model—GMG Linear Equation Solver Package Documentation. No. 2004-1261. 2004. 
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