computationally impractical. The solution is to define a different ordering, such that the and are linear combinations of the original annihilation and creations operators. The combinations are chosen to ensure that the thermal expectation values of normal ordered products are always zero so the split chosen will depend on the temperature. References F. Mandl, G. Shaw, Quantum Field Theory, John Wiley & Sons, 1984. S. Weinberg, The Quantum Theory of Fields (Volume I) Cambridge University Press (1995) T.S. Evans, D.A. Steer, Wick's theorem at finite temperature, Nucl. Phys B 474, 481-496 (1996) arXiv:hep-ph/9601268 Quantum field theory