"Molecule release is unable to find surface upon which to place molecule"

I set up a basic experiment in 2D to test Mcell. I defined a particle that is going to diffuse in an square. When I tried to fix the positions of the molecules in this square the mcell software give me this error:

Warning: Molecule release is unable to find surface upon which to place molecule t.
  This could be caused by too small of a SITE_DIAMETER on the release site 'World.t_rel'.
Warning: Molecule release is unable to find surface upon which to place molecule t.
  This could be caused by too small of a SITE_DIAMETER on the release site 'World.t_rel'.
Warning: Failed to release 2 molecules from list "World.t_rel" at iteration 0.
Warning: during the simulation some reaction probabilities were greater than 1. You may want to rerun the simulation with the WARNINGS block enabled to get more detail.

The mdl file of the geometry is:

Square POLYGON_LIST
{
  VERTEX_LIST /* in microns */
  {
    [ 0.00000000000000, 0.00000000000000, 00000000000000 ]
    [ 0.00000000000000, 1.00000000000000, 00000000000000 ]
    [ 1.00000000000000, 1.00000000000000, 00000000000000 ]
    [ 1.00000000000000, 0.00000000000000, 00000000000000 ]
  }
  ELEMENT_CONNECTIONS /* Define triangles */
  {
    [ 0, 1, 2 ]
    [ 0, 2, 3 ]
  }
  DEFINE_SURFACE_REGIONS
  {
    top
    {
      ELEMENT_LIST = [0, 1]
    }
  }
}

and the mdl file of the experiment is:

ITERATIONS = 100
TIME_STEP = 1E-5

/* Define molecules. The units for diffusion are cm^2 s^(-1) */

DEFINE_MOLECULES{
  a {
    DIFFUSION_CONSTANT_2D = 1E-6
    }
  t {
    DIFFUSION_CONSTANT_2D = 0
    }
}

INTERACTION_RADIUS=0.005

/* The rate is defined inside the brackets */
DEFINE_REACTIONS {
  /* annihilation of a */
  a' + t' -> t' [1e8] : annih
}
/* Some files could be useful to be included separately */

INCLUDE_FILE = "./Square.mdl"

INSTANTIATE World OBJECT {
  Square OBJECT Square{}

  a_rel RELEASE_SITE {
  SHAPE = World.Square[top]
  MOLECULE = a'
  NUMBER_TO_RELEASE = 2000
  RELEASE_PROBABILITY = 1
  }
  t_rel RELEASE_SITE {
  SHAPE = LIST
  RELEASE_PROBABILITY = 1
  SITE_DIAMETER = 100 /* for some strange reason this is given a releasing error */
  MOLECULE_POSITIONS
  {
    t' [ 0.20000000000000, 0.20000000000000, 0.00000000000000 ]
    t' [ 0.20000000000000, 0.80000000000000, 0.00000000000000 ]
    t' [ 0.80000000000000, 0.20000000000000, 0.00000000000000 ]
    t' [ 0.80000000000000, 0.80000000000000, 0.00000000000000 ]
  }
  }
}

Thanks in advance!

mcellteam / mcell

"Molecule release is unable to find surface upon which to place molecule" #30