TCEC Season 18

The 18th season of the Top Chess Engine Championship began on 4 May 2020, and is currently ongoing. The defending champion is Leela Chess Zero, which defeated Stockfish in the previous season's superfinal.

Overview

For this season, there are five leagues: the Qualification League (QL), League Three, League Two, League One, and Premier Division. Two engines promote in every league, with the top two engines of the Premier Division contesting a 100-game superfinal. Updates are allowed only between League Three and League Two, between League One and Premier Division, and between Premier Division and the superfinal. In a small change to previous seasons' rules, draw adjudication now occurs if the evaluations of both engines are within +/- 0.15 for five consecutive moves, after move 35. This increases the threshold of evaluation compared to +/- 0.08 for the previous season.

In contrast to previous seasons, only active engines that are stronger than the best humans are invited to participate. The addition of the "active" criterion means that several veteran competitors, such as former champions Jonny and Houdini, are not participating. Further, a more stringent uniqueness criterion meant that PeSTO (which shares a search algorithm with rofChade) and Komodo MCTS (which shares an evaluation algorithm with Komodo) are not participating.

Tiebreak rules

The tiebreak rules for TCEC Season 18 are:

1. For any event except the superfinal: the number of crashes.
2. Head-to-head score.
3. Number of wins.
4. Sonneborn-Berger score.
5. A decision by the tournament organizers.

Results

Qualification League

Ten engines participated in the Qualification League. Seven engines returned from the previous year, Monolith returned after a two-season hiatus, and two new engines (Combusken and Weiss) made their first appearances. The league turned out to be very closely contested: every engine lost at least two games, and in the final standings, only 2 points separated winner Counter from 7th-placed Combusken. This meant that seven engines were in with a chance up until the final games. Eventual 2nd-place finisher Asymptote staged a mini-comeback after a poor first round to finish second, ahead of 3rd-placed Monolith on the back of a head-to-head win. chess22k, the only engine that had promoted out of QL previously, started well but stalled in the second half and ended in 4th place. Tail-enders Bagatur and Weiss were heavily handicapped by only being able to make use of one thread while their competitors usually ran on 176, and finished with only 5/18 and 1.5/18 respectively.

Pos	Engine	Pld	W	D	L	Pts	Qualification
1	Counter	18	9	6	3	12	Advance to League 3
2	Asymptote	18	8	7	3	11.5
3	Monolith	18	8	7	3	11.5
4	chess22k	18	6	10	2	11
5	ChessFighter	18	6	9	3	10.5
6	FabChess	18	6	8	4	10
7	Combusken	18	5	10	3	10
8	Tucano	18	2	10	6	7
9	Bagatur	18	3	4	11	5
10	Weiss	18	0	3	15	1.5

Source: [1]

League Three

Ten engines participated in League 3. Every engine here played in Qualification League last year, and while the top and bottom engines were unchanged, the order of the middle engines saw some shifting. Demolito took advantage of S17 QL winner Defenchess leaving for greener pastures to win League 3 with 12/18. Second-place was closely contested, with Igel taking a 1-point lead after 60 games, only to see Gogobello storm from behind with a string of victories, including an impressive win as black over Demolito, and place second by 0.5 points. Meanwhile Pirarucu, which finished ahead of both Igel and Gogobello last year, placed a disappointing 6th, though it did hand Gogobello its only loss. On the other end, QL promoters Counter and Asymptote found League 3 far more unwelcoming and relegated back to QL with 7/18 and 6/18 respectively. Counter initially looked like it could hold on, starting out with 10 straight draws, but finished with 4 losses and 4 draws in its remaining games, becoming the only engine not to win a game.

Pos	Engine	Pld	W	D	L	Pts	Qualification
1	Demolito	18	7	10	1	12	Advance to League 2
2	Gogobello	18	5	12	1	11
3	Igel	18	4	13	1	10.5
4	Minic	18	4	11	3	9.5
5	iCE	18	4	10	4	9
6	Marvin	18	2	14	2	9
7	Topple	18	2	12	4	8
8	Pirarucu	18	2	12	4	8
9	Counter	18	0	14	4	7	Relegate to QL
10	Asymptote	18	1	10	7	6

Source: [2]

League Two

Booot, Chiron, and ChessBrainVB, which did not play in the previous season because they did not run on Wine, all returned to play this season in League Two. Among the competitors three engines quickly distanced themselves from their rivals: Booot, Pedone and Winter. Booot was comfortably in the lead for most of the division and was the first to promote, while Winter trailed after losing a game against Pedone. In one of the final games of the league, Winter scored a win against Booot, which put it half a point ahead of Pedone; however, it was all for naught as Pedone drew its game in hand against Chiron, which tied the scores between the two and let Pedone qualify on tiebreak.

At the bottom of the table, in a repeat of League Three, the two promoted engines Demolito and Gogobello found the competition much stiffer and were both relegated back to League Three.

Pos	Engine	Pld	W	D	L	Pts	Qualification
1	Booot	18	8	9	1	12.5	Advance to League 1
2	Pedone	18	6	12	0	12
3	Winter	18	7	10	1	12
4	Vajolet	18	3	12	3	9
5	Chiron	18	3	11	4	8.5
6	Wasp	18	3	11	4	8.5
7	ChessBrainVB	18	1	13	4	7.5
8	Nemorino	18	2	11	5	7.5
9	Demolito	18	2	10	6	7	Relegate to League 3
10	Gogobello	18	1	9	8	5.5

Source: [3]

League One

Fire, which had performed respectably in last season's Premier Division in spite of not having been updated for over a year, won League One. Along with last season's promotion rival rofChade, it did not lose a game the entire league, with both finishing comfortably above third-placed Booot. In contrast, the middle of the table was fiercely contested. Booot was the only engine to score two wins against its mid-table rivals, defeating Fritz and ScorpioNN, but it lost a game to last-place Pedone. Defenchess lost a game to Xiphos, but it along with Fritz was able to 2-0 Pedone, thereby taking 4th and 5th respectively. Last season's Premier Division engine ScorpioNN, as well as League One contender Xiphos, both performed solidly and lost only one game; however they also only scored one win which was insufficient to fight for the top spots. At the other end of the table, promoted engine Pedone was quickly outclassed, losing eight games. For the other relegation spot, both Arasan and RubiChess failed to score any wins, but Arasan lost only one game, leaving RubiChess to relegate.

Pos	Engine	Pld	W	D	L	Pts	Qualification
1	Fire	18	5	13	0	11.5	Advance to Premier Division
2	rofChade	18	4	14	0	11
3	Booot	18	3	13	2	9.5
4	Defenchess	18	3	13	2	9.5
5	Fritz	18	2	14	2	9
6	ScorpioNN	18	1	16	1	9
7	Xiphos	18	1	16	1	9
8	Arasan	18	0	17	1	8.5
9	RubiChess	18	0	15	3	7.5	Relegate to League 2
10	Pedone	18	1	9	8	5.5

Source: [4]

Premier Division

In the Premier Division, Stoofvlees (which had not been updated since Season 17) took a surprising lead after the first round robin. It recorded the only double-kill (winning with both sides of the same opening) against rofChade, and by the end of the first round robin, was on +4 while pre-tournament favorites Stockfish, Leela and AllieStein were only +2. However, the lead did not last. Stockfish played a sterling second round robin, including a win against both Stoofvlees and AllieStein, to take first place. Leela also closed the gap by not losing any games while defeating Ethereal, rofChade and Fire. By the third round robin, Stockfish was effectively qualified for the superfinal, while the last remaining slot was still up for grabs. Leela effectively eliminated Stoofvlees after the latter blundered a simple tactic (diagram). However, AllieStein was only a win behind, and had superior tiebreakers to boot. Leela defeated Ethereal, only for AllieStein to keep pace by beating Komodo. It looked as though Stoofvlees' blunder might've been decisive to Leela qualifying for the superfinal. The question was finally settled in game 164 when AllieStein lost the reverse game to Komodo, which qualified Leela for the superfinal. Both Leela and first-placed finisher Stockfish did not lose a game in the division.

Stoofvlees–Leela Chess Zero

	a	b	c	d	e	f	g	h
8									8
7									7
6									6
5									5
4									4
3									3
2									2
1									1
	a	b	c	d	e	f	g	h

In this equal position, Stoofvlees played 47. Kf2??, losing immediately to 47...Kg6. Black pins and wins the f3-knight because 48. Ke3 loses to 48...Rxf3+ 49. Kxf3 d2 50. Ke2 Bc4+, forking and winning the rook on b5.

Pos	Engine	Pld	W	D	L	Pts	Qualification
1	Stockfish	42	9	33	0	25.5	Advance to Superfinal
2	Leela Chess Zero	42	7	35	0	24.5
3	AllieStein	42	7	33	2	23.5
4	Stoofvlees	42	7	32	3	23
5	Komodo	42	4	34	4	21
6	Ethereal	42	0	38	4	19
7	rofChade	42	1	30	11	16	Relegate to League 1
8	Fire	42	0	31	11	15.5

Source: [5]

Superfinal

The superfinal is currently in progress.