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How strong are these programs?
Official site:
http://www.tcec-chess.org/stage_1.php
All the strongest programs (Houdini, Rybka ++) are also participating.
These are the strongest programs avaiable to the public, except Fritz and Thinker for technical reasons.
Another interesting participant to add would be Rondo (follow-up engine of Zappa) but it’s still private. Zappa Mexico II is there and performs well.