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The FIX statements is used in the METHODs part of a model to constrain particular variables in the model. A variable which has been fixed has a known value assigned to it, and ASCEND will not attempt to change that value. The FIXed variables in a model are used as the basis from which the remaining un-FIXed (FREE) variables can be calculated. By default, no variables are FIXed until an explicit command to FIX them is given.

(* add this to the example on the FIX page *)
METHOD solve_x;

    (* reconfigure the model to solve for x such that y=0 *)
    FREE x;
    FIX y;
    y:= 0;

END solve_x;

Note that variables can also be FIXed and FREEd in an ad-hoc way, using the ASCEND GUI.

See also FREE.