International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. B, ch. 3.4, p. 390

The incomplete gamma function may be expressed in terms of commonly available functions such as the exponential integral and the complement of the error function. The definition of the exponential integral is The definition of the complement of the error function is Numerical approximations to these functions are given, for example, by Hastings (1955). The recursion formula for the incomplete gamma function (Davis, 1972) may be used to obtain working formulae starting from the special values of and which are defined above. Also we note that .
References
Davis, P. J. (1972). Gamma function and related functions. Handbook of mathematical functions with formulas, graphs, and mathematical tables, edited by M. Abramowitz & I. A. Stegun, pp. 260–262. London, New York: John Wiley. [Reprint, with corrections of 1964 Natl Bur. Stand. publication.]Hastings, C. Jr (1955). Approximations for digital computers. New Jersey: Princeton University Press.