Even though the Candidates' tournament took place in Berlin, the most popular opening was... the Catalan! Common sense requires organizing the next edition in Barcelona so that the players could feel free to play the Berlin Ruy Lopez. But leaving joke aside, following my databases on the Catalan opening, I have made an overview of how the Candidates treated it during this historic tournament. I have mainly focused on the games which are relevant to our repertoire but made shorter comments on all the other games as well.
Game 1 - Caruana, F - So, W
This was the system prepared by Caruana for this tournament. In several similar positions, he developed his queen to b3 instead of the usual Qc2.
In my database Play the Catalan - Complete Repertoire for White I suggest 8.Qc2.
Game 2 - Ding, L - Caruana, F
The system with 7...b6 is becoming very popular recently. No surprise that Caruana decided to give it a try. In the arising positions, Black obtains a very unusual compensation for the exchange.
In the database Play the Catalan - Complete Repertoire for White - Part 2 I suggest 7.Qa4. My subsequent analysis shows that in that case, the move 7...b6 is less effective.
Game 3 - Caruana, F - Karjakin, S
In this game, Caruana opted for this queen sortie even before making a castle. Nevertheless, soon the game transposes to the normal lines.
Game 4 - Ding, L - Mamedyarov, S
Nowadays, the move 8.a4 is much more popular than 8.Qxc4. Nevertheless, as I have demonstrated in Part 2 of my Catalan Repertoire, 8.Qxc4 gives White excellent chances to retain some pressure.
Game 5 - Mamedyarov, S - Karjakin, S
Once again, Karjakin went for 7...c6 which is played with the obvious intention to meet 8.Qxc4 with 8...b5. Mamedyarov decided to grab the c4-pawn with a knight and opted for 8.a4 followed by 9.Nbd2. In this way, he gave Black just enough time to consolidate.
Game 6 - Caruana, F - Ding, L
In this encounter, the Chinese GM met Caruana's favorite 8.Qb3 with 8...b6. The idea is to make use of the fact that White doesn't have the resource b2-b3 at his disposal. Nevertheless, Caruana proved that in the arising symmetrical structure after 9.Nc3 Ba6 10.cxd5 cxd5, White retains some pressure.
Game 7 - Aronian, L - Mamedyarov, S
This is another game which features the fashionable 7...b6. Instead of the ambitious 8.Ne5, Aronian played to modest 8.Qxc4. After carrying out the thematic c7-c5 advance, Black gradually equalized completely.
Game 8 - Mamedyarov, S - Caruana, F
In this game, Caruana opted for the rare 7...Be7 which was not covered in my Catalan repertoire. Serious analysis proves that this move is a hard nut to crack. It seems that in the main line Black has good chances to keep the balance.
Therefore, in my annotations to the game, I suggest 8.Qe2 instead of 8.Nfd2 which was suggested in my theoretical database.
Game 9 - Grischuk, A - Karjakin, S
In this game, Grischuk tried the rare 9.Rc1 in a well-known position. White relies on the fact that in the long run, Black cannot avoid ...b7-b6, in order to complete his queenside development and then the control over the c-file after cxd5, ... cxd5 would he useful for White.
Karjakin, however, was well prepared for this type of position and neutralized White's pressure with a couple of accurate moves.
In Part 1 of my Catalan Repertoire, I suggest 9.Rd1 followed by 10.b3.
Game 10 - So, W - Mamedyarov, S
In this game, we see Mamedyarov playing Karjakin's pet line with 7...c6 followed by 8...b5. As I pointed out in my annotations, the move 9.Qc2 which was played by Wesley So fails to create problems for Black. In my opinion, White should go for 9.Qd3.
Game 11 - Aronian, L - Karjakin, S
We have the same system as in the previous game. This time, however, the position arose via different move order. The queen stands a bit better on b3 than on c2, as it keeps the b5-pawn under observation, causing some problems with the thematic ...c6-c5. As mentioned before, my favorite is Qd3 but this was not available with the given move order.
Game 12 - Kramnik, V - Mamedyarov, S
This is the only game in the tournament in which White refrained from the traditional 7.Qc2. Instead, Kramnik played 7.Ne5 to which Mamedyarov reacted with the unusual 7...Qd6!?. Kramnik treated the position in an original way and had chances to put pressure on Black's position.