On Nov 27, 2007, at 13:43 , Ezad Shojaee wrote: > i have a silly problem about Si lattice. I defined it as an SC > (simple cubic) > with 8 atoms per basis: > > ATOMIC_POSITIONS > Si 0.00 0.00 0.00 > Si 0.50 0.50 0.00 > Si 0.00 0.50 0.50 > Si 0.50 0.00 0.50 > Si 0.25 0.25 0.25 > Si 0.75 0.75 0.25 > Si 0.25 0.75 0.75 > Si 0.75 0.25 0.75 > > K_POINTS > 4 > 0.1250000 0.1250000 0.1250000 1.00 > 0.3750000 0.3750000 0.3750000 1.00 > 0.3750000 0.1250000 0.1250000 3.00 > 0.3750000 0.3750000 0.1250000 3.00 > > the Total Energy is 4-times as great as the Fcc structure > total energy (till the 5-th digit), but the code has detected > 24 symmetries, with no inversion for the SC one. What's > the problem(if there is any)? Should'nt be 48 symmetries?
there isn't any problem, but there is a subtlety. The 24 symmetries that have no associated fractional translation are found; the remaining 24 have an associated fractional translation (1/4 1/4 1/4) and are also found, but not accepted. The reason is that in a supercell the choice of fractional translations is not unique and you have no (simple) way to be sure that the set of all symmetry operations is actually a group, in the mathematical sense. Paolo --- Paolo Giannozzi, Dept of Physics, University of Udine via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222
