05 Dec 21
In Petrosian-Taimanov (Zurich Candidates, 1953), the following position was reached after 15...dxe4.
Here, White played 16. Qg4, with the better game. In his book on the tournament, David Bronstein says that 16. d5 cxd5 17. Nb5 "would be worth trying, threatening Nc7... and Nd6..."
17...Nc5 would seem the critical reply, allowing Black's queen to control the d6-square, while shielding the c7-square from White's rook. (See next diagram for this sequence.)
White would then have a host of attacking ideas, the most direct being 18. Nh6+, 18. Nxg7, and 18. Bxg7.
A) 18. Nh6+ gxh6 19. Qg4+ Qg5 20. Qxg5+ hxg5 21. Nc7, and the material White is about to regain will be insufficient.
B) 18. Nxg7 Bxg7 19. Qg4 is met by 19...f6.
C) 18. Bxg7 Bxg7, and now
C1) 19. Qg4 is answered by 19...Qf6.
C2) 19. Nbd6 (attempting to the exploit the deflection of Black's bishop from the a3/f8 diagonal) fails against 19...Re6.
C3) 19. Nc7 (attempting to deflect the queen from where she could reach the f6-square) allows her to protect the g7-bishop along the second rank after 19...Qxc7 20. Qg4 f6.
Creating a battery along the a1/h8 diagonal by 18. Qd4 would threaten 19. Nxg7 or 19. Nh6+ Kh8 20. Nxf7+. But both threats can be thwarted by 18...f6, and 19. b4 Ne6 (19...Nd3 might work, but 19...Ne6 protects the sensitive c7-square and practically forces additional material gain.) 20. Qd2 (20. Qc3 would meet the same reply.) 20...Ba6 21. a4 Qd7 22. Nfd4 Nxd4 would soon net Black a second extra pawn. (See next diagram for this sequence.)
Sacrificing White's b5-knight at c7 would deflect Black's queen from the h4/d8 diagonal, but White's attack would stall after 18. Nc7 Qxc7 19. Nh6+ (19. B(or N)xg7 Bxg7 20. Qg4 f6; 19. Qd4 f6; 19. b4 Qd7 20. Qg4 g6) 19...Kh8 20. Qh5 Re6 21. Nxf7+ Kg8 22. Ng5 Rh6. (See next diagram for this sequence.)
White's b5-knight might instead be sacrificed at d6 to deflect Black's bishop from the defense of the g-pawn, but this too would fail: 18. Nbd6 Bxd6 19. Nxg7 (19. Bxg7 Re6, to meet 20. Qg4 by 20...Rg6) 19...Re5 20. Bxe5 Bxe5 21. Nf5 Qg5. (See next diagram for this sequence.)
18. b4 would give White access to the c7-square. But it seems to favor Black after 18...Nd3 (next diagram):
A) 19. Bxg7 Bxg7 20. Qg4 Qf6 21. Rc7 Bc8 22. Nbd6 Bxf5 23. Nxf5 Re5, or 20. Nxg7 Kxg7 21. Qg4+ Kh8 22. Rc7 Re7. (See next diagram for the first sequence.)
B) 19. Nc7 Nxb2 20. Qg4 g6 21. Nxa8 Qxa8 (threatening 22...Bc8 23. Qe2 Nd3) 22. Qg3 Nc4 (to block the file before re-centralizing Black's queen). (See next diagram for this sequence.)
C) 19. Rc7 Nxb2 20. Qg4 Re6 21. Rxb7 a6 22. Nbd4 (22. Nc7 would meet the same reply) 22...Rg6 23. Qf4 Nd3 24. Qc7 Qxc7 25. Rxc7 Rf6 (25...Bxb4 26. Nc6, and the threat of 27. Ne7+ gives White time to trade Black's bishop and double rooks on the c-file; 25...Nxb4 would allow 26. Rfc1 and Rc8, with Ne7+ in the air), preparing ...g6 followed by bringing the f6-rook back into the game via the d6-square. (See next diagram for this sequence.)
D) 19. Qg4 Re6!, to answer 20. B(or N)xg7 by 20...Rg6.
Finally, 18. Qg4 Re6 would likely transpose to one of the above continuations.
Conclusion: 16. d5 cxd5 17. Nb5 would have given Black the edge after 17...Nc5.