Classical spin system. Ising model.

LagrangeEuler

Energy function ##E=-S_1S_2##. I took ##J=1##. If spin are oriented paralel energy is negative. How could energy be negative?

LagrangeEuler	Classical spin system. Ising model. Energy function ##E=-S_1S_2##. I took ##J=1##. If spin are oriented paralel energy is negative. How could energy be negative?