nullmove

Nullmove is a technique used in computer chess to improve the search efficiency of the alpha-beta algorithm, a common search algorithm used in game tree search.

In alpha-beta, nodes in the search tree are evaluated using a heuristic estimate of the node's value. When exploring the search tree, the algorithm uses the concepts of alpha and beta to prune branches and limit the search to potentially interesting parts of the tree. However, when the algorithm encounters a position where the best move is a draw, or a "null move", the evaluation of the move attempts to take into account the move that leads to a draw, rather than an actual move that would change the board.

In order to deal with nodes that lead to a nullmove, some implementations incorporate a special way of evaluating the nullmove by treating it as if the opponent's move was decided before the current player's move. This approach slightly complicates the alpha-beta algorithm but avoids reevaluating all the moves in the subtree, thus increasing efficiency.

Some more advanced techniques, like Killer Heuristic and Late Move Prune, are usually combined with Nullmove to achieve a better search performance.

Computer chess programs that implement null move technique include Stockfish, Houdini, and Leela Chess Zero, among others. The use of null move can lead to significant improvements in search performance for good players.