Keano,

I don't think there is anything magical about 21...a5 in the bishop sac line.  It's just that the soundness of the sacrifice seems to hang on a knife's edge - in order for it to succeed, White must actually get a bit of help from a bad defensive choice by Black on the 21st move.  For example:

• 21...g6 fatally opens the long diagonal.
• 21...Nf8-g6 loses time 

In order to defend, Black's king needs only to run to the center.  As near as I can tell from my analysis, 21...a5 gets queenside play going enough to tip the balance in Black's favor.

As for 19...a5, there a particular problems which seem to make the move unsuitable.  One of these is the weakness of the b5-pawn.

whiteatak shredder,

Thanks for your condolences regarding 19.Bxh7+. I agree it would have been an interesting investigation, and I was VERY tempted to play it even when I couldn't find a good continuation for White in the (19...Kxh7 20.Qh3+ Kg8 21.g5) 21...a5 line.  Maybe we can take another look at this later if there is time.

I think you're right about 19...g6 looking like the best choice.  In addition to 19...a5, which looks doubtful, I also briefly considered 19...Nf8 and 19...Bxc3 (which just looks wrong in principle) and wasn't impressed with either.

There may be more to 19...Nf8 than I think (I would probably answer with 20.f5 if this helps), but I doubt it will prove better than 19...g6.

Cheers,
Ostap

whiteatak shredder,

My move is 19.Kb1

1.e4 e6 2.d4 d5 3.Nc3 Nf6 4.e5 Nfd7 5.f4 c5 6.Nf3 Nc6 7.Be3 cxd4 8.Nxd4 Bc5 9.Qd2 0-0 10.0-0-0 a6 11.Nb3 Bb4 12.Bd3 b5 13.g4 Bb7 14.Rhg1 Na5 15.Rg3 Nc4 16.Qe1 Qh4 17.Bd4 Rfc8 18.Qf1 Qe7 19.Kb1




After taking a serious look at 19.Bxh7+ Kxh7 20.Qh3+ Kg8 21.g5 I'm fairly convinced that 21...a5 is a viable defense, and there may be others (although I don't place 21...g6, 21...Nf8 or 21...Ba3 this category).

It is with some sadness that I give up on the bishop sac, but I'm reasonably happy with White's prospects after 19.Kb1 - which was the move (after 18...Qe7) on which I based by decision to play 18.Qf1 in the first place.

Here's my analysis of 21...Nf8 (with Keano's and MNb's lines noted)

21...Nf8 22.Qh5 Ng6 23.Rh3 Qc7

This is the best place for Black's queen

Keano’s line: 23...Qd7 24.f5! exf5 25.Qh7+ Kf8 26.a3 Bxc3 27.Bxc3 a5 28.Nxa5 Nxa5 29.Bb4+

23...Qe8 also loses to 24.a3 or 24.f5 

24.Qh7+ 

MNb’s line: 24.f5 exf5 25.Qh7+ (25.e6!?) 25...Kf8 26.a3 Bxc3 (26...Nxa3!?) 27.Bxc3 Ncxe5

After 24.f5 exf5 White could try 25.a3 (iso 25.Qh7+ Kf8 when I can't find a serious improvement for White.) 25...Bxc3 26.Bxc3 Ncxe5 (26...Kf8?? 27.Bb4+ Ke8 28.e6 Nd6 29.Nd4 Kd8 30.Re1 Qb6 (after 30...Bc6 31.exf7 Qxf7 32.Bxd6 White recovers the piece with an overwhelming position.) 31.exf7 Ne4 (31...Qxd4 32.Qxg6+-) 32.Rxe4 dxe4 33.Qxg6 Qxg6 34.f8Q+ Kc7 35.Rc3+ and Black gets mated) 27.Qh7+ Kf8 28.Na5 




28...Rab8 29.Nxb7 Qxb7 (29...Rxb7 30.Bb4+ Ke8 31.Rc3+-) 30.Bxe5 Nxe5 31.Qh8+ Ke7 32.Qxg7+-




24...Kf8 25.a3




25...Nxa3

Only move: 25...Bxc3? loses to 26.Bxc3 a5 27.Nxa5! (regaining control of b4) 27...Nxa5 (27...Qxa5 prevents White's next, but so does resigning.) 28.Bb4+ Ke8 29.Qg8+ Kd7 30.Qxf7+ Kd8 




31.Rh8+! The final sacrifice, setting up a short-lived king hunt. 31...Nxh8 32.Qf8+ Kd7 33.Qe7+ Kc6 34.Qxe6+ Qd6 35.Qxd6#

26.bxa3 Bxa3+ 27.Kb1 Ke8 28.f5! exf5 29.Qg8+ Nf8




30.e6! looks pretty devastating for Black.
(30.Re1 also looks strong.)

Furthermore after 20.g5 all of a sudden 21.g6+ is a threat.

MNb wrote on 01/12/07 at 01:06:57:

Interesting you should single out this knight as evidence of disharmony in White's position, because the one line I am having trouble dealing with at the moment starts off with 21...a5!  Harassment of the b3-knight seems to give Black some surprisingly annoying counterplay. So far this move seems to have the best potential to kill the bishop sac idea, although I haven't given up hope just yet...

I think I may have convincing answers to 21...g6 (which actually seems to lose quite quickly), 21...Nf8, and even 21...Ba3.  The last of these leads to fairly complicated play, and I could easily have missed something in the analysis.  I'm at work at the moment (and have to keep this brief), but I will try to post something tonight on these three moves.

I will also, time permitting, try to explain why 21...a5 is giving me such headaches at the moment.

BTW:

• I agree that it is too early to call the bishop sac promising.  However, the idea has a lot more potential than my intuition suggested when I first considered it.
• The bishop sac is exactly the kind of move that engines will miss because, immediately after 19....Kxh7, evidence of White's compensation is beyond the computer's search horizon.  Analyzing from the position after 21.g5, however, Fritz 9 eventually starts to see seemingly plausible defenses by Black go south.

Keano and MNb: Thanks for the responses!  I think this is a really interesting line of play to analyze even if it eventually has to be rejected as not quite sound.

My last post for now is that 21...Ba3!? is possible attempting counter-attack, but it not clear to me how Black continues after for example the calm 22.Nc3-e2. Will be interested to here some other thoughts on this continuation, for now it looks very promising for White to me.

yes the bishop sac could be an interesting attempt - my first thought was that after 19.Bxh7+ Kxh7 20.Qh3+ Kg8 21.g5 then 21...g6 intending ...Qf8-g7 would defend.

But now I am a bit concerned by 22. Rg4!? intending 22...Qf8 23. f5!! exf5 24. Rh4 Qg7 25. e6! and things have gone pear-shaped have they not? I thought you guys with engines said this move was not on