A rational approximation of the Voigt function

Rapid and accurate computation of the Voigt function

This function file is a subroutine for rapid and accurate computation of the Voigt function. It covers the domain of practical interest 0 < x < 40,000 and 10^-4 < y < 10^2 required for applications using the HITRAN molecular spectroscopic database. The average accuracy in this domain is 10^-14. Use opt = 1 for more accurate and opt = 2 for more rapid computation. By default opt = 1.