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Sorry for the delay of the answer. I don’t know why locations in Croatia are missing in the database, but we will try to investigate this. The method to calculate the solar irradiation (normal beam and horizontal diffuse) from the clearness number is an older, more general method described e.g. in the 2005 ASHRAE Handbook - Fundamentals, chapter 31. This method was kept for the user-defined design days calculation in IDA ICE and ESBO, since the user might not know the clear-sky optical depth parameters. The differences between the two methods makes it not easy to convert between them. The best I could recommend at the moment is to either use the user-defined method as it is or to pick a site in a nearby country that is available in the database. For dry places a clearness number larger than 1 is recommended, while for humid locations, a clearness number smaller than 1 is recommended. For examples in the U.S., see Fig. 5 in chapter 33 in the 2007 ASHRAE Handbook – HVAC Applications. All the required data for the user-defined method is shown in the data input form.