Here's something that caught my eye in the mental martial art category (i.e. the Kung Fu of Chess), as a very high-level example of the importance and efficacy of mental calmness:
Right from the opening [GM HIkaru Nakamura] obtained an edge, and after a couple of mistakes by the Czech [GM David Navara], it was sac-sac-mate. When asked whether the rest day had been the source of the pixie dust, he responded that in fact it was his game against Anish Giri. For some inexplicable reason he had left that game feeling at peace, in a mood where chess just seems easy, and even regretted the rest day, anxious to play then and there.
(The original article on round 5 of the 2012 Tata Steel tournament in Wijk aan Zee can be found here on the ChessBase site.)