Nature Communications 4: Article number: 2272 (2013); Published: 12 August 2013; Updated: 30 September 2013; Updated: 22 November 2013.

Typographical errors were inadvertently introduced into equations (3)–(6) during the production process, such that the term T, representing temperature, is missing from the denominator of some exponents. These errors do not affect the analysis of results presented in the paper. The correct equations follow.