Arcanum is a free UCI chess engine under the MIT license.
Author:Lars Aurud, Rating Chess Engines Diary CEDR=3323
Arcanum 2.7 - what's new?
Performance Changes
Add a beta-cut to singular extension if there are other good moves (not singular), and the singular extension beta-margin is larger than beta.
Reduce counter moves less in LMR
Reduce less in LMR when in a PV node.
Add continuation history and use it in move ordering.
Add static pruning using SEE.
Add lazy updating of the NNUE accumulator. This increases performance by removing unused updates of the accumulator.
Change the NNUE architecture from 768->8x(512->16->1) to 768->1024->1. The network is faster and has more weights due to the larger feature transformer. A new net follows this update to the architecture: arcanum-net-v6.0.fnnue. The new net is embedded in the executable, so the separate net is not required to use the engine.
Remove TB probing in quiet search.
Allow queen promotion in quiet search.
Some minor improvements to move generation.
Bugfixes and Other Functional Changes
Remove the feature setting the search depth to 1 when only a single move was available.
Remove terminating search when a mate is found. Thus it continues to search for shorter mates. Mate distance pruning is still used.
Allocate a number of tables on the heap to avoid large allocations on the stack. Be more efficient with the memory allocated on the stack. At high plies, the previous version could crash due to exceeding the stack size.
Fix wrong usage of getKnightMoves() in hasEasyCapture() which potentially could cause crashes.
Include castle rights as a part of the Zobrist hash. This ensures that castle rights are considered when checking of repeated positions.
Note that Arcanum still requires AVX2 and does not support pondering
Improving ELO/Playing Strength
Arcanum v2.7 is estimated to be 76.3 Elo stronger than Arcanum v2.6 based on self play.
Self Play
Time control: 120s + 1s
Opening book: 8moves_v3.pgn
Tablebase: 5-man syzygy.
Score of v2.7 vs v2.6: 225 - 52 - 523 (Win - Loss - Draw)
v2.7 playing White: 138 - 15 - 247
v2.7 playing Black: 87 - 37 - 276
Elo difference: 76.3 +/- 13.8, LOS: 100.0 %, DrawRatio: 65.4 %
Arcanum 2.6 vs other engines:
| PlentyChess 6.0.2 | 6/46 | -34 | 46 Games |
| Artemis 17.3TR sl | 4.5/46 | -37 | 46 Games |
| Saturn 1.3 JA | 13/41 | -15 | 41 Games |
| Frozenight 7.0.0 JA | 13.5/29 | -2 | 29 Games |
| Astra 6.1.1 | 2.5/29 | -24 | 29 Games |
| Juggernaut 1.01 | 28/28 | +28 | 28 Games |
| Pawnocchio 1.8 | 3/28 | -22 | 28 Games |
| Tactical Titan v2 | 0.5/27 | -26 | 27 Games |
| Critter 1.6a | 19.5/26 | +13 | 26 Games |
| Axolotl 1.9 JA | 21.5/22 | +21 | 22 Games |
| ShashChess 39 | 1.5/22 | -19 | 22 Games |
| Clover 8.2.1 | 2.5/18 | -13 | 18 Games |
| Pawnocchio 1.8.1 | 1.5/12 | -9 | 12 Games |
| Cadie 2.1 | 6/11 | +1 | 11 Games |
| Oxide 1.4.0 | 7/10 | +4 | 10 Games |
| Potential 2.0.0 JA | 6.5/10 | +3 | 10 Games |
| Zahak 10.0 JA | 4.5/10 | -1 | 10 Games |
| Uralochka 3.42 dev14 | 2/10 | -6 | 10 Games |
| Yuliana 7.0 | 1.5/10 | -7 | 10 Games |
| Lc0 0.32.0-rc1 | 1.5/10 | -7 | 10 Games |
| CorChess 20250702 | 1.5/10 | -7 | 10 Games |
github:https://github.com/LarsAur/Arcanum/releases/tag/2.7
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