The GCG: Generalized Conjugate Gradient Solver pane is is on the MODFLOW Packages and Programs dialog box under MT3DMS or MT3D-USGS. It is used to specify some of the data for the GCG solver in MT3DMS. The GCG solver is required for all MT3DMS models.
MXITER is the maximum number of outer iterations; it should be set to an integer greater than one only when a nonlinear sorption isotherm is included in simulation.
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ITER1 is the maximum number of inner iterations; a value of 30-50 should be adequate for most problems.
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ISOLVE is the type of preconditioners to be used with the Lanczos/ORTHOMIN acceleration scheme:
= 1, Jacobi
= 2, SSOR
= 3, Modified Incomplete Cholesky (MIC) (MIC usually converges faster, but it needs significantly more memory)
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NCRS is an integer flag for treatment of dispersion tensor cross terms:
= 0, lump all dispersion cross terms to the right-hand-side (approximate but highly efficient).
= 1, include full dispersion
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ACCL is the relaxation factor for the SSOR option; a value of 1.0 is generally adequate.
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CCLOSE is the convergence criterion in terms of relative concentration; a real value between 10-4 and 10-6 is generally adequate.
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IPRGCG is the interval for printing the maximum concentration changes of each iteration. Set IPRGCG to zero as default for printing at the end of each stress period.
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