S matrix poles and the second virial coefficient

For cutoff potentials, a condition which is not a limitation for the calculation of physical systems, the S matrix is meromorphic. It may be expressed in terms of its poles, and then the quantum mechanical second virial coefficient of a neutral gas may be calculated. This paper takes another look at this approach, and discusses the feasibility, attraction and problems of the method. Among concerns are the rate of convergence of the ‘pole’ expansion and the physical significance of the ‘higher’ poles.