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If the difference is small, do not panic. It is quite normal for
iterative methods to reach convergence through different paths as soon
as anything changes. In particular, between serial and parallel
execution there are operations that are not performed in the same
order. As the numerical accuracy of computer numbers is finite, this
can yield slightly different results.
It is also normal that the total energy converges to a better accuracy
than its terms, since only the sum is variational, i.e. has a minimum
in correspondence to ground-state charge density. Thus if the
convergence threshold is for instance 10-8
, you get 8-digit
accuracy on the total energy, but one or two less on other terms
(e.g. XC and Hartree energy). It this is a problem for you, reduce the
convergence threshold for instance to 10-10
or 10-12
. The
differences should go away (but it will probably take a few more
iterations to converge).
Next: 7.23 the CPU time
Up: 7 Troubleshooting
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Paolo Giannozzi
2009-10-01