Events

An event in COPASI represents a discrete, conditional state transition in your model. Each event must have two components: a trigger, which causes the event to occur, and at least one assignment, which modifies the model. If an event contains multiple assignments, they are executed simultaneously (regardless of the order in which they are listed). Additionally, an event can include an optional delay, specifying how long COPASI will wait after detecting the event trigger before carrying out the assignments.

Trigger

An event trigger is a Boolean expression. The event is fired at the exact moment when the value of the expression changes from FALSE to TRUE. Note that the Boolean trigger expression cannot contain any of COPASI’s random functions (uniform(), gamma(), poisson(), or normal()). By default, the trigger only activates during a FALSE to TRUE transition. To allow the trigger to fire at the initial simulation time if the expression is already TRUE, enable the Fire at initial Time if Trigger is true option.

Priority

If multiple events trigger at the same time, you can use a priority expression to specify which event should execute first.

Assignment

An event assignment consists of two elements: an expression and a target. All assignments defined for a single event are applied together as a unit. This means COPASI will update all relevant model components at once, and only then check for further events. To perform an assignment, COPASI evaluates your expression and assigns the resulting value to your specified target.

Delay

An event delay is optional and defines the amount of simulation time between firing the event and executing its assignments. There are two ways to apply a delay:

1. Delay Calculation and Assignment: The delay occurs between the trigger and calculation of the target expression. If selected, the value to be will be calculated and assigned only after the delay when triggered.
2. Delay Assignment Only: The delay occurs between calculating the expression and assigning the value to the target object. If selected, the value to be assigned will be calculated when triggered, but that value will be assigned after the delay.

If you use a delay, you have the option Trigger must remain true. If this is enabled, COPASI will only execute the event if the trigger expression remains true for the entire duration of the delay.

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In the model tree, the Events branch is found directly below Global Quantities. Selecting the Events branch opens a table listing all events in your model. For a new model, this table will be empty. You can use the search field to filter the displayed events: only events matching your filter in any column will be shown, making it easy to find events by name or target.

If you click on the name of an event in the tree on the left or double click on a row of the table the detailed information correlated with the chosen event will be displayed.

On the event detail screen, you can specify the Boolean trigger expression, an optional delay, and multiple event targets. When adding an event target, COPASI will only allow you to select values that are not already determined by assignments, as modifying such values would create conflicts.

For each mathematical expression associated with an event—including the trigger, delay, and assignment expressions—you can use the same elements available for defining user functions. For a detailed description of these elements, see the User Defined Functions section.

You can also reference values from other model entities in your mathematical expressions. These can be conveniently selected by pressing the button with the COPASI icon.