[Pw_forum] Titanium Molecular Dynamics Stability Issues

[Nicholas Walker]

Dear All,

I am trying to run molecular dynamics simulations (Born-Oppenheimer and Car-Parrinello) of titanium at various temperatures using position and velocity input data from LAMMPS. I have successfully run such simulations with aluminum, but titanium has proved troublesome thus far. It seems that no matter what I do, the first conjugate gradient step experiences a missed minimum, converges, and then diverges on the second step (with ortho going bananas).

So far, I have tried reducing the timestep (to 1-5 au), increasing the number of max iterations for ortho (50-100), Gram-Schmidt orthogonalization, reducing the energy convergence threshold (10^-6), and two different pseudopotentials (pz-hgh and pz-sp-hgh), and different configurations from classical code. None of these variations have changed the result. Each time, there is a missed minimum (case 3) in CG before the first step converges followed by a second step that diverges.I even tried using the converged first step as a starting point for Car-Parrinello MD, but that still fails to converge due to ortho going bananas.

Can anyone help me understand what is going wrong and how I might fix this problem?

Thanks,
Nicholas Walker
Louisiana State University
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