I read this story a long time ago, and have been unable to find it again. Xander Harris is actually the youngest Shepphard son, and somehow winds up at the family estate. He has his own room/area, where he tinkers with computers and other things. He creates some kind of super computer that will track and compile whatever info he needs.
Earth comes under some kind of attack, and people are evacuated first to The Mountain, and then to Atlantis. All the Shepphard's are evacuated (including Xander) and later they somehow access his computer to find out what happened on earth.
And that's literally all I remember. I don't even know if the story was finished or not. Does anyone have any ideas?
FOUND - story is Finding Home by Draconia. Link is http://archiveofourown.org/works/411794
Thank you so much - and wow, was that fast!