##C=\frac{1}{N}\sum_{\vec{k}} \frac{J(0)}{J(0)-J(\vec{k})} ## ##J(\vec{k})## is exchange integral in ##\vec{k}## space. What is the name of this constant and where I can find more about it? For simple cubic lattice ##C_{SC}=1.516##
Can you provide a bit more context? Exchange integrals usually depend on details of the band structure, so I am surprised that you can get a universal constant for all simple cubic lattices irrespective of lattice constant, atomic flavor etc. J(0) would favor ferromagnetism J(k) with k != 0 would favor antiferromagnetism with a modulation wave vector k.