Please see items 11.7.0.8, 11.7.0.9 and 11.7.0.11 of the user guide: http://www.quantum-espresso.org/user_guide/ node60.html#SECTION000127080000000000000
P. --- Paolo Giannozzi, Dept of Chemistry&Physics&Environment, Univ. Udine, via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222
