### what is the meaning of contact energy's Positive value and Negative value

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8 months ago by
Assume there are two cells are contacted，and set contact energy J to a negative value. Does it means those two cells will definitely stay together ， if set J to a positive value does it means those two cells possible to separate and its probability is decided by the value of J.
Community: CompuCell3D
Hi, I am new in CC3D, can we have a communication in Chinese? My QQ is 1775755174
written 8 months ago by Yuan

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8 months ago by
Usually, besides the j value between a pair of cell types you also need to take into account the j values for the two cell types with medium.

Besides that, you always need to take into account all possible interactions. So even if a particular pair of cells has a j value of -10 and a large contact area (contact energy is J*contact_area) that contact could still be displaced if either of the two cells has a higher affinity (more negative J) for some other cell type and/or the other cell type is more flexible and can acquire a larger contact surface area.

So, in general, you need to take into account all possible pairs of interactions; for N cell types (and assuming Medium is one of those N types) there are N^2/2 + N/2 = (N/2)(N+1) possible interactions. For 3 cell types there are 6 interaction, for 4 cell types there are 10 interactions, ...

Beyond the J values the actual interaction energy will also be dependent on how stiff the surface of the cells is. With a very low LambdaSurface and/or large target surface area, cells can create extra surface in order to reap the reward of a large contact surface if that lowers contact energy.

Finally, you need to remember that movement of a CC3D configuration to a lower energy configuration can only occur if a suitable pathway exists. In other words, it is possible to trap a CC3D system in an energetically unfavorable state if there is no reasonably low energy pathway to a lower energy state. (That is also true of biology, biological systems are trapped in high energy states (life) since a low energy pathway to lower energy states (death) isn't always readily available.)
Regardless of other conditions，does it means the relationship of contact strength is J=-10>J=-5>J=0>J=5>J=10？
And the meaning of J=0 is confused for me.
written 8 months ago by dali Zan
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Yes, that is correct the j=-10 interaction is stronger than the j=10 one. A j of zero just means there is no contribution to the systems total energy due to that particular interaction. If all your j's are either (for example) +10 or zero then the system will try to get all the contacts to be of the zero type. If all your j's are -10 and zero then the system will try to get all the contacts to be of the -10 type.
written 8 months ago by James Sluka