The calculation of magnetic response functions in crystals

This paper presents a method for calculating the real part of the noninteracting paramagnetic generalized susceptibility function, Re chi 0 (q, omega ). The accuracy of the method is to better than 1%, and any errors are systematic rather than statistical. The structure of the curves and the singularities in the first derivative are clearly defined. The method takes the principal-parts nature of the integration fully into account. The numerical accuracy of the method is discussed at length, and illustrated by calculations of Re chi 0 (q,0) at T=0 for a single face-centred cubic, tight-binding s band. A brief discussion of previous work is also given.