Tuesday, May 26, 2015

Calculating Forcing Moves: Defensive Idea 3, Part 2 (Prepare to Move)

When your opponent creates a (real) threat, or you are considering creating one against your opponent, there are five defensive ideas to consider.  We have already looked at different ways to implement the first two defensive ideas:  Idea 1 (capture the attacking piecepin the attacking piecedeflect the attacking piece), or Idea 2 (block the attacking piece).  We also explored the third basic idea of moving out of the attack.

In this post we will look at a position where we "prepare to move".  These moves are possible when the threat is one move away, and you have time to move another piece out of the way so that you can escape to, or through, that square when the threat is played.

The general process is to evaluate the position, identify and prioritize all threats, and then to use the five defensive ideas to identify specific candidate moves.  Next, calculate each candidate move to the end and evaluate the final positions (material and activity).  As you progress through the candidates, keep in mind the "king of the hill" (KOTH) candidate move to help you pick the best at the end.

The below position is from Ward Farnsworth's wonderful chesstactics.org website, but the analysis here is my own!  We'll follow the same approach as usual:  evaluate the position, identify and prioritize all threats, use the five defensive ideas to identify specific candidate moves, and calculate each candidate to find the best (or least bad) reply.

Black just played ...Qe7.  White to move.

5k2/pb2qp1p/1p6/6R1/3p4/8/PP4PP/R2N3K w - - 0 1

Let's evaluate the position and then find all of black's threats.  Black is up a queen and a pawn vs two rooks, and black's pieces and passed pawn are significanly more active than white's pieces.  Black is playing for a win and white is fighting to draw.  Black has two concrete threats:  checkmate (...Qe1#) and win a rook (...Qxg5).  Of those two threats, the checkmate is the priority.  Let's look at the five defensive ideas to see what candidate moves we find, and then pick the best move.  Hopefully we can avoid checkmate AND save the rook.

Idea 1 (capture, pin, deflect):  none.
Idea 2 (block):  1. Re5 and 1. Ne3 directly block the queen from getting to e1, while 1. Rf5 "prepares to" block on the next move with Rf1.
Idea 3 (move):  since the mate is one move away we can look at direct king moves, as well as moves that create an escape square ("prepare to move").  The king only has one move, but 1. Kg1 does not allow the king to escape checkmate because f2 will be covered by the queen on e1.  White can also "prepare to move" his king by creating an escape square on h2 (the bishop is taking away g2), by playing 1. h3 or 1. h4.
Idea 4 (defend):  white can defend the mating square on e1 with his rook on a1, if the knight can get out the way.  We already have 1. Ne3 from above, which we now see might accomplish two things at once (block the queen, and allow the rook to defend e1).  The knight has two other possible squares to move to 1. Nc3 and 1. Nf2.
Idea 5 (counterattack with an equal or greater threat to WIN, DRAW, DEFEND):  there is not time to create equal threat.  A greater threat would have to start with a check, and white has 1. Rg8+.

The five defensive ideas have given us eight actual candidate moves to calculate further:  1. Re5, 1. Ne3, 1. Rf5, 1. h3, 1. h4, 1. Nc3, 1. Nf2, and 1. Rg8+.  Let's start with white's most forcing candidate move 1. Rg8+:

1...Kxg8 (only).  White has lost a rook and has no more checks.  He therefore cannot force a draw by perpetual and is not able to reposition his other pieces to defend using the other four ideas.  Reject 1. Rg8+.

None of the remaining seven moves seem particularly more forcing than the others, so let's just go through them in the order that we discovered them.  1. Re5:

1...Qxe5 appears to be black's most forcing reply.  Black is again threatening mate with no new defensive options available and after 1. Nf2 black is ahead a queen and a (passed) pawn vs a rook, so 1...Qxe5 appears to refute white's candidate.  Reject 1. Re5.

The other block we found also defended e1, so let's next look at the interesting 1. Ne3:

1...Qxg5 appears to be black's most forcing reply, so let's see if that is a refutation.  White is now down more material and black again has two concrete threats:  to capture the knight and, should the knight move, to deliver checkmate with Qxg2#.  Simply pushing the d pawn also looks like a fun attacking idea for black, and would give white a third threat to have to defend against.  Saving the knight (and defender of g2) is white's first priority so let's look at the five defensive ideas and see if any work.  White cannot capture, pin (2. Rd1 doesn't work), deflect, or block the pawn or queen.  He can only move the knight to two squares that do not immediately allow mate (2. Ng4, and 2. Nd5).  He can defend the knight with 2. Re1 but, after 2...dxe3 the rook cannot recapture on e3 because of the mate threat on g2.  That only gives us two defensive ideas, both of which just delay the mate in 1 by one more move.  Then after the obligatory 3. Rg1, black's two threats (passed pawn and checkmate) will make for a simple win, and we can safely say that 1...Qxg5 refutes white's candidate.  Reject 1. Ne3.

The last blocking idea we found prepares to block on f1 after Qe1+, but seems easily refuted.  After 1. Rf5:

1...Qe1+ 2. Rf1 (only) Qxf1#.  Yup, easily refuted.  Reject 1. Rf5.

Next up would be the two candidate moves that create an escape square for the white king, and prepare for him to move out of the check.  Let's look first at 1. h3:

1...Qe1+ is black's most forcing move, but after the king moves away with 2. Kh2, black has no clear follow-up.

1...Qxg5 appears stronger for black, since he is now up even more material and is threatening checkmate on the next move with ..Qxg2.  After considering all five defensive ideas, we find whites' only move to prevent the mate is 2. Ne3 following by 3. Rg1.  So while 1...Qe1+ does very little for black, 1...Qxg5 is a crushing refutation.  Reject 1. h3.

The other block seems much more interesting because it also defends the rook on g5, so let's see if black has anything left after 1. h4:

1...Qe1 (again black's most forcing move) 2. Kh2 (only) Qxh4+ 3. Kg1 (only) Qxg5 4. Ne3 (forced) dxe3 5. Kf1 (only way to prolong...) Qxg2+ 6. Ke1 Qf2+ 7. Kd1 (only) Qd2#.  Reject 1. h4.

The final two candidate moves (1. Nc3 and 1. Nf2) come from the idea of defending the e1 mate square.  Since 1. Nc3 puts the knight en prise, let's see what we get with 1. Nf2:

1...Qxg5 (most forcing) 2. Rg1 (forced) and now Qd2 appears simple and wins at least the b pawn (defending the pawn with 3. Nd1 allows Bxg2+ 3. Rxg2 (only) Qxd1+ 4. Rg1 (only) and now black has won a pawn and traded the minor pieces but also has ...Qf3+ followed by ...d3.).

White's last defensive hope is 1. Nc3:

1...Qxg5 (most forcing) 2. Rg1 (forced) and now black just picks off the c3 knight, making this line worse than 1. Nf2.

After looking at eight candidate moves, we can confidently resign knowing that we have not missed a single defensive opportunity for white.  If we felt compelled to play on for some sadistic reason, the two least bad moves would be either 1. Rg8+ or 1. Re5, which "only" leave black ahead a queen and passed pawn vs a rook.  All other moves are worse than that!  In future posts I will discuss ways to track multiple evaluations in your head while calculating, including positions where all of your choices are relatively bad or your best move is only slightly better than the second best!

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