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I am Black and my opponent is White. Here the computer recommends Rxd4, which I have done (the rook captured bishop here).

[FEN "4r1k1/1pp2pp1/p6p/4q3/P2rPn2/2P1QP1P/1P4P1/R4RK1 w - - 0 1"]

It's obvious then that Qxd4 is a blunder, because then I play Ne2 to fork the king and queen and the game is over. I would expect the computer to play cxd4 here to go up material, but the computer plays Rfe1.

Why not cxd4 here? What future sequence am I not seeing?

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    My first instinct would be cxd4 Qg5, threatening Qxg2# and Nxh3+ (the latter winning the queen). Not computer checked though. – koedem Jan 13 at 22:13
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    I checked SF12 and it thinks cxd is best for White. Still lost after ...Qg5 though – B.Swan Jan 13 at 22:21
  • SF12 is the open source engine Stockfish. – Timo Jan 15 at 14:40
  • @koedem: I think your instinct is right, I would only check a second if White has anything (Rf2, Qf2, Qd2 - nope, the only move is Kf2, but then Qxg2 followed by Qxb2, curtains.) Qg5 is usually a motive against an undefended Qd2, but here it works wonders too. – Hauke Reddmann Feb 14 at 20:00
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Stockfish, at depth 20, evaluates Rfe1 with a score of -8.5, while cxd4 has one of -2.6. With so large a difference, the explanation is that you were playing against a weak computer. There is usually a gradient (e.g. from 1 to 10) of difficulty to choose from when playing against a computer, so you have probably chosen something far from 10. Chess.com (guessing from the screenshot) also has adaptative opponents that will grow stronger/weaker according to your performance.

For what it's worth, shortly after cxd4 White gives up a rook and a pawn for Black's knight and has an exposed king and piteous pawn structure —so Black wins anyway—, as follows.

4r1k1/1pp2pp1/p6p/4q3/P2rPn2/2P1QP1P/1P4P1/R4RK1 w - - 0 6

1. cxd4 Qg5 2. Kf2 Qxg2+ 3. Ke1 Qxb2 {Double threat: Qxa1+ and Ng2+ (a royal fork)} 4. Qxf4 Qxa1+ 5. Ke2 Qxd4

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