The way I remember it, he thought for 50 minutes after 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6, and then finally played 4.Bxc6. He had to think so long "because I was about to weaken 32 light squares".
Anyway I think Carlsen just wanted better concentration.
Why Carlsen look stressed while playing first move?
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Kramnik claimed kasparov wants Carlsen to play 1.e4 ....but Carlsen finally played d4 after thinking for whole 2 minutes...but Carlsen had lot of time before to decide which first move to play ....so i find it odd Carlsen thinking so much for first move