Abstract
We develop a graphical method for computing the virial expansion coefficients for a nonrelativistic quantum field theory. As an example we compute the third virial coefficient b3 for unitary fermions, a nonperturbative system. By calculating several graphs and performing an extrapolation, we arrive at b3=-0.2930, within 0.7% of a recent computation b3=-0.290 952 95 by Liu, Hu, and Drummond, which involved summing 10,000 energy levels for three unitary fermions in a harmonic trap.
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