// Last edited on 2019-04-05 18:06:12 by stolfi

#macro halo(ctr,rad,Q,i)
  // Halo for an ion at center {ctr}, radius {rad}, charge {Q}.
  // Let's try a halo volume equal to a constant {Vc} times the atom volume:
  #local Vc = 1.0;
  #local Va = 4*pi/3 * rad*rad*rad;
  #local Vh = Vc * Va;
  #local rh = cbrt(Vc + 1.0)*rad;n
  // We want the color of the halo near the atom to be proportional to {Q}.
  // If the halo is emission/extinction the color will be 
  #local L = 2*sqrt(rh*rh - rad*rad); // Length of halo chord near atom. 
  #local 
  #local s = sqrt(sqrt(abs(Q))); 
  #local rh = (1 + (i+1.0)/abs(Q))*rad; // Radius of halo.
  #if (Q > 0)
    #local color = < 1.000, 0.700, 0.400 >;  // Limiting color of positive halo.
  #else
    #local color = < 0.400, 0.700, 1.000 >;  // limiting color of negative halo.
  #end