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Subsection 3.3.1: Control Parameters Up Section 3.3: Control Section Subsection 3.3.3: Common Solver Parameters

### 3.3.2 Time Stepper parameters

The optional time_stepper element sets the parameters that control the FEBio auto-time stepper. This auto-time stepper will adjust the time step size depending on the convergence stats of the previous time step. It defines the following parameters.
 Value Description Default dtmin Minimum time step size (1) dtmax Maximum time step size (3) opt_iter Optimal, or desired, number of iterations per time step (2) 10 max_retries Maximum number of times a time step is restarted. (2) 5 aggressiveness Algorithm for cutting the time step size after a failed time step 0
1. The dtmin and dtmax values are used to constrain the range of possible time step values. The opt_iter defines the estimated optimal number of quasi-Newton iterations. If the actual number of iterations is less than or equal to this value the time step size is increased, otherwise it is decreased.
2. When a time step fails (e.g. due to a negative jacobian), FEBio will retry the time step with a smaller time step size. The max_retries parameter determines the maximum number of times a timestep may be retried before FEBio error terminates. The new time step size is determined by the ratio of the previous time step size and max_retries+1. For example, if the last time step size is 0.1 and max_retries is set to 4, then the time step size is adjusted by 0.02: The first retry will have a step size of 0.08; the next will be 0.06, and so on.
<dtmax lc="1">0.1</dtmax>