A design objective is a logical expression that consists of two mathematical expressions separated by one of the logical operators: ==, <, <=, > or >=. These are called the left-hand-side (LHS) and right-hand-side (RHS) mathematical expressions and both must have computable numerical values. They may contain any combination of numbers, constants, variables, standard or custom output parameters as well as EM.Cube's legitimate functions. Objectives that involve the logical operator "'''<nowiki>==</nowiki>'''" are regarded a "'''Goals'''". The RHS expression of a goal is usually chosen to be a number, which is often known as the "'''Target Value'''". In the logical expression of a goal, one can bring the two RHS and LHS expressions to one side establish an equality of the form "(LHS - RHS) == 0". Numerically speaking, this is equivalent to minimizing the quantity | LHS - RHS |. During an [[optimization]] process, all the project goals are evaluated numerically and they are used collectively to build an error (objective) function whose value is tried to be minimized. Objectives that involve "non-Equal" logical operators are regarded a "'''Constraints'''". Unlike goals which lead to minimizable numerical values, constraints are rather conditions that should be met while the error function is being minimized.
To define an objective, open the '''Objectives Dialog''' either by clicking the '''Objectives''' [[Image:objective_icon.png]] button of the '''Simulate Toolbar''', or by selecting '''Menu > Simulate > ''''''Objectives...''' from the Menu Bar, or using the keyboard shortcut '''Ctrl+J'''. The objectives list is initially empty. To add a new objective, click the '''Add''' button to open up the '''Add Objective Dialog'''. At the bottom of this dialog, you can see a list of all the available EM.Cube output parameters including both standard and custom output parameters. This list may vary depending on the types of sources and observables that you have already defined in your project. You can enter any mathematical expressions in the two boxes labeled '''Expression 1''' and '''Expression 2'''. The Available Output Parameter List simply helps you remember the syntax of these parameters. You should also select one of the available options in the dropdown list labeled '''Logical Operator'''. The default operator is '''"== (Equal To)"'''. As soon as you finish the definition of an objective, its full logical expression is added to the Objective List. You can always modify the project objectives after they have been created. Select a row in the Objective List and click the '''Edit '''button of the dialog and change the expressions or the logical operator. You can also remove an objective from the list using the '''Delete '''button.
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