Hi,guys, I am not sure am I supposed to ask for help here or not. But I do need your guys' help to give me some hint solving this problem. I have a parallel fortran code which is written by us. It seems like working well but actually the output from 1 node to 5 nodes are the same(the results I think are correct).When I am using more than 6 nodes, the output is different,which I think is wrong. For example,the following lines are the output from 1(or 2) node calculation and 6 nodes calculation. Look at the data. The atomic positions are the same. But the order of atoms are different. Why dose this happen? Why the results are different with 5 nodes and 6 nodes? atom type atomic position 1 Al 0.0000 0.0000 -1.8869 2 O -1.6539 0.1249 -1.0842 3 O 0.7188 -1.4947 -1.0842 4 O 0.9351 1.3699 -1.0842 5 Al 2.3727 -1.3699 -0.2816 6 Al -2.3727 1.3699 0.2816 7 O 1.6539 -0.1249 1.0842 8 O -0.7188 1.4947 1.0842 9 O -0.9351 -1.3699 1.0842 10 Al 0.0000 0.0000 1.8869 1 Al 0.0000 0.0000 -1.8869 2 O -1.6539 0.1249 -1.0842 3 O 0.7188 -1.4947 -1.0842 4 Al 0.9351 1.3699 -1.0842 5 O 2.3727 -1.3699 -0.2816 6 O -2.3727 1.3699 0.2816 7 O 1.6539 -0.1249 1.0842 8 Al -0.7188 1.4947 1.0842 9 Al -0.9351 -1.3699 1.0842 10 Al 0.0000 0.0000 1.8869 Your help is really appreciated, Wayne