**Acknowledgments**

Albert Rimola is indebted to Ministry of Economy (MINECO) of the Spanish Government for a Juan de la Cierva contract. Financial support from MINECO (projects CTQ2013-40347-ERC and CTQ2011-24847/BQU) and from Department of Economy of the Catalan Government (project 2009SGR-638), and the use of the Catalonia Supercomputer Centre (CESCA) are gratefully acknowledged. Mariona Sodupe also acknowledges support through the 2011 ICREA Academia award.
