set facet tension ... command.
         The contribution to the total energy is the sum
         of all the facet areas times their respective surface
         tensions. The surface tension of a facet may also be specified
         as depending on the phases 
     of the bodies it separates.
     In the string model, the
     tension resides on edges instead of facets.
Example datafile: cube.fe
  E = \int\int\int_{body} G \rho  z  dV 
         but is calculated using the Divergence Theorem as
  E =  \int\int_{body surface} G\rho {z^2\over 2} \vec k \cdot \vec{dS}.  
 
         This integral is done over each facet that bounds a body.
         If a facet bounds two bodies of different density, then
         the appropriate difference in density is used.  Facets
         lying in the z = 0 plane make no contribution, and may be
         omitted if they are otherwise unneeded.  Facets lying
         in constraints may be omitted if their contributions to
         the gravitational energy are contained in 
constraint energy integrals.
         In the string model,
         all this happens in one lower dimension.
Example datafile: mound.fe
           E = \int_{edge} F . dl. 
  
 The integrand is defined in the 
constraint declaration in the datafile.
	 The integral uses the innate orientation of the edge,
	 but if the orientation attribute of the edge is negative,
	 the value is negated.
         This is useful for prescribed contact angles on walls
         (in place of wall facets with equivalent tension)
         and for gravitational potential energy that would
         otherwise require facets in the constraint.
         The mound example illustrates this.
Example datafile: ringblob.fe
  E = k\left\Vert \vec S \times \vec Q \right\Vert / 6
     
         where \vec S is the edge vector and \vec Q is the 
         projection of the edge on the tangent plane 
         of the constraint at the tail vertex of the edge.
         The constant k is a global constant called the "gap
         constant".  A gap constant of 1 gives the best
         approximation to the actual area of the gap.  A larger
         value minimizes gaps and gets vertices nicely spread
         out along a constraint.  You can set the 
         value of k in the 
datafile or
 with the k command.
The gap energy falls off quadratically as the surface is refined. That is, refining once reduces the gap energy by a factor of four. You can see if this energy has a significant effect on the surface by changing the gap constant.
Note: gap energy is effective only in the linear model.
Example datafile: tankex.fe
         E = P*V_0*ln(V/V_0)  
where P is the ambient pressure, V_0 is the target volume of the body,
and V is the actual volume.  To account for work done against the
ambeint pressure, each body also makes a negative contribution of 
E = -P*V.The ambient pressure can be set in the datafile or with the p command. This energy is calculated only for bodies given a target volume.
Example datafile: crystal.fe