Problems With The Null-move Heuristic
There are a class of chess positions where employing the null-move heuristic can result in severe tactical blunders. In these zugzwang (German for "forced to move") positions, the player whose turn it is to move has only bad moves as their legal choices, and so would actually be better off if allowed to forfeit the right to move. In these positions, the null-move heuristic may produce a cutoff where a full search would not have found one, causing the program to assume the position is very good for a side it may in fact be very bad for.
To avoid using the null-move heuristic in zugzwang positions, most chess-playing programs that use the null-move heuristic put restrictions on its use. Such restrictions often include not using the null-move heuristic if
- the side to move is in check
- the side to move has only its king and pawns remaining
- the side to move has a small number of pieces remaining
- the previous move in the search was also a null move.
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