Additional Sources of Free Energy
In some cases, the expression for the free energy can include other sources. It is possible to incorporate these by modifying Equation 11-16:
(11-18)
where f is a user-defined free energy (SI unit: J/m3).
The expression for the external free energy must be manually differentiated with respect to ϕ and then entered into the f/∂ϕ field. In most cases, the external free energy is zero.