# 13.1. Time Step Control Commands

The larger the time step is, the greater the mass scaling. Part of the Cylinder With Legs test input file invokes POD and controls the way that it works.

    BEGIN PARAMETERS PRESTO REGION
      #USER TIME STEP = 1.2e-08
    END PARAMETERS PRESTO REGION

    BEGIN PROPER ORTHOGONAL DECOMPOSITION
      NUMBER OF POD MODES = 21
      SNAPSHOTS INTERVAL = 405
      POD MODES COMPUTATION TIME STEP = 1.e-4
      ENERGY PERCENTAGE = 99.999
      POD FILTER = On
      MODE REFRESH = Off
      #USER FILTER TIME STEP = 1.2327e-08
      USER FILTER TIME STEP = 6.1635e-08 #5X
    END PROPER ORTHOGONAL DECOMPOSITION

The `USER TIME STEP` overrides the element time step, including any increase in the time step due to POD.

The initialization needs the `NUMBER OF POD MODES` for post-processing. It is the number of fields to allocate that will store POD modes. The way that POD is set up at this time, this is also the number of snapshots and the number of eigenvalue eigenvector pairs.

`SNAPSHOTS INTERVAL` is the number of time steps taken between snapshots used to build the correlation matrix. It is not the number of time steps between adding a POD vector.

`POD MODES COMPUTATION TIME STEP` is the time at which Adagio POD is activated.

`ENERGY PERCENTAGE` is percentage that the sum of the eigenvalues used for the ECM/POD run accounts for with respect to the sum of the total eigenvalue spectrum. It is related to the kinetic energy of the system, but is not the ECM energy percentage [[1]].

`POD FILTER` activates the high frequency mass scaling. If `OFF`, then the simulation is equivalent to a simulation without ECM/POD.

`MODE REFRESH` This refreshes the number of POD modes throughout the simulation.

`USER FILTER TIME STEP` This is the user defined time step.

In theory the Lanczos algorithm provides the time step, but this has not been implemented.

The Plastic Cylinder test of POD activates the `MODE REFRESH`.

    MODE REFRESH=ON

Snapshots are stored throughout the simulation. At every `POD MODES COMPUTATION TIME STEP`, 1e-4 seconds here, the POD modes are updated using the new information.