subsection Material model
  set Model name = inner core material

  # The 'inner core material' model also contains a function that
  # represents the resistance to melting/freezing at the inner core
  # boundary.
  # For P-->infinity, the boundary is a free slip boundary, and for
  # P-->0, the boundary is an open boundary (with zero normal stress).
  subsection Inner core
    subsection Phase change resistance function
      set Variable names      = x,y,z
      set Function expression = 1e-2     # <-- P
    end
  end
end
