the class `NagaiHondaForce`

https://github.com/Chaste/Chaste/blob/release/cell_based/src/population/forces/NagaiHondaForce.cpp#L133

calls `GetAreaGradientOfElementAtNode`

(https://chaste.cs.ox.ac.uk/public-docs/VertexMesh_8cpp_source.html#l01423) which in its definition has a difference_vector that is being multiplied by 0.5 and -0.5.

Also, the area_gradient[0] gets the value in difference_vector[1] and area_gradient[1] gets difference_vector[0] after multiplication. In short the rationale of line 1439-1440 in https://chaste.cs.ox.ac.uk/public-docs/VertexMesh_8cpp_source.html#l01423 is unclear. Someone please highlight the rationale. thanks !