Energy Conservation Problems

[August Melcher]

Hello all,

I've recently been working on implementing B97M-V, a new functional from 
Narbe Mardirossian in the Martin Head-Gordon group, in LIBXC for use with 
CP2K. I have in fact done so and been able to reproduce the Q-Chem binding 
energies on the S22 dataset up to a threshold we have deemed to be 
reasonable based on sample calculations with PBE-D3 and rVV10. However, I 
am running into trouble when moving to the condensed phase. I have 
attempted to simulate a 64 water molecule cluster with PBC but I see large 
amounts of drift in the energy. The attached plots show this drift. The 
first shows the drift over several ps, the second shows what the behavior 
looks like initially, over ~150 fs, and the last shows that oddly if I run 
without the local correlation term, with just exchange and the nonlocal 
rVV10 correlation, energy appears to be conserved. A sample input file is 
attached (BLYP pseudopotentials were found to give optimal performance on 
S22 for B97M-V).

In attempting to rectify this problem, I have tried (without success) the 
following:
-Increasing the PW cutoff to 5000 Ry
-Using the QZV3P basis set
-Trying a different initial condition
-Running without ASPC, using other WF guess methods including previous rho, 
previous WF, PS, and linear PS
-Using a 0.25 fs timestep instead of 0.5 fs
-Using an SCF convergence threshold of 10-8 instead of 10-7
-Using the following smoothing/derivatives combinations: NN50/PW, 
NN50/SPLINE3, NN4/NN4, NN10/NN10

Does anyone have any idea what may be going on here? I only made changes to 
the LIBXC code, so perhaps I neglected to update some code which calculates 
the density derivatives in CP2K. Also the list of smoothing/derivative 
combinations is by no means exhaustive, so perhaps I just haven't found the 
proper combination for the numerics of this functional.

Thanks,
August
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