[QE-users] HSE XC - mismatch between energy- and force-minimizing bond length for H2 and O2

[Robert Wexler]

Dear QE users,

I obtained some perplexing results using the HSE XC functional that I would
like to share with you.

I calculated the ground state potential energy surfaces of H2 and O2 as a
function of bond length strain (i.e. % different from some arbitrary
starting bond length). The results are attached in this email as
"h2-compare-force-energy.pdf" and "o2-compare-force-energy.pdf",
respectively. The right panel for H2 and O2 shows the total energy as a
function of bond length strain. As you can see, a slight contraction of the
original bond length is preferred. If you look at the left panel, however,
the total force is minimized at positive strains of greater magnitude. The
minimum force and total energy should occur at the same bond length strain,
which could present some problems for geometry optimization. I have
attached inputs and outputs for each calculation in .tar.gz files
("h2-qe-in-out.tar.gz" and "o2-qe-in-out.tar.gz").

I took this one step further and tried to relax a water molecule using HSE
and found that the relaxation does not converge. This can be seen in
"h2o-hse-relax.pdf". As the BFGS proceeds, it cannot converge because one
configuration decreases the total energy (steps 1, 4-12, 15-16) and another
decreases the total force (steps 2-3, 13-14).

Here is a quick summary of my computational resource and QE settings:
- Cori, NERSC, KNL
- QE 6.3
- GBRV PSPs
- SCF calculation
- 35/350 Ry cutoffs
- LSDA for O2 (I get the correct total/absolute spin magnetization of the
O2 molecule)
- HSE, 1x1x1 q-grid
- Assume isolated, "mt"
- Gamma point
- More details can be found in the inputs and outputs I've supplied

Thank you in advance for any help you can provide.

Best,
Rob
---------------------------------------------
Robert B. Wexler, PhD
University of Pennsylvania, 2019
rwexler at sas.upenn.edu
(215) 801-8741
---------------------------------------------
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