This parameter determines how many complete calculations of the temperature and ionization structure of the model shell are made. Multiple passes are needed because there is no a priori knowledge of the optical depth of the shell in all the lines and continua, and these can affect the state of the gas in the interior of the shell. During the first pass the calculation proceeds through the shell, and assumes that all optical depths from points within the shell to the far edge of the shell are 0. If an integer greater than 1 is supplied as a parameter, XSTAR performs that number of iterations through the entire calculation, setting the optical depths to the far edge at the values calculated in the previous iteration. The odd numbered passes are made from the smallest to largest radius, while the even numbered passes are made in the inward direction. The emergent spectrum is not calculated accurately during the inward passes, so npass must be odd. Multi-pass calculations substantially improve the accuracy of the predictions made for shells with finite thickness, but they are much more time consuming than single-pass calculations. They also make use of temporary unformatted datasets, named 'xout_tmp.lis', 'xout_tmp2.lis', which can become quite large. The default value for this parameter is 1.