Dear Jeremy
It is not a technical problem, it is a theoretical one.
negative ions in gas phase generally yield unbound excess electrons
leading to weird results when computed at the GGA level of theory. Of
course water stabilizes OH-, and also exact exchange if you use a
hybrid functional.
HTH
Giuseppe
Quoting RABONE Jeremy <[email protected]>:
Hello,
I have just started a project using QE, and have a question
regarding a hydroxide ion in a box. When I first tried to do a
relaxation with conv_thr = 1.0d-8, it started off like it was going
to converge and then suddenly the convergence went all over the
place. Changing all of the other parameters made no difference, but
reducing
conv_thr to 1.0d-6 stops the scf step before the problem and the
calculation completes. There also seems to be some effect of the
randomised starting wavefunctions even with conv_thr = 1.0d-6
(sometimes converges, sometimes goes all triplets). When surrounding
the ion with a water cage, the calculation works with no problems.
If I switch to a spin polarized calculation, with conv_thr = 1.0d-8
it seems determined to converge to a triplet state with the first
antibonding orbital occupied, and does so with no difficulty is the
magnetization is set to 2.
I recall there were sometimes issues in DFT when the order of state
energies changes during a calculation - is this what is happening
here and is there a way to get passed the problem or do I just have
to be content with a reduced threshold ?
A S2- ion in a box does something similar - seemingly converging to
triplet with a d-state occupied, and even when surrounded by a water
cage it fails to converge in a singlet state (although in this case
one might question the validity of the calculation).
I'm using SG15 norm-conserving pseudopotentials but the PAW
pseudopotential seems to do the same thing.
Regards,
Jeremy
GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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E-mail: <[email protected]>
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