Los Alamos chess

Los Alamos chess (or anti-clerical chess) is a chess variant played on a 6×6 board without bishops. This was the first chess-like game played by a digital computer program. This program was written at Los Alamos Scientific Laboratory by Paul Stein and Mark Wells for the MANIAC I computer in 1956. The reduction of the board size and the number of pieces from standard chess was due to the very limited capacity of computers at the time. The computer still needed about 20 minutes to compute a move. The program was very simple, containing only about 600 instructions. It was mostly a minimax tree search and could look four plies ahead. For scoring the board at the end of the four-ply lookahead, it estimates a score for material and a score for mobility, then adds them. Pseudocode for the chess program is described in Figure 11.4 of Newell, 2019. In 1958, a revised version was written for MANIAC II for full 8×8 chess, though its pseudocode was never published. There is a record of a single game by it, c. November 1958 (Table 11.2 of Newell, 2019). According to a 2023 study by Roger Sayle, Los Alamos chess has about 4*10^29 possible positions, and a branching factor of about 22. This makes it considerably simpler than standard chess (possible positions: ~8.7*10^45, branching factor: 35), but still more complex that games like checkers and Othello/Reversi, which have been weakly solved.