[QE-users] cell_dofree with BFGS relaxation

[Daniel Marchand]

Thanks again. So for the BFGS output, for whatever reason there is a final
SCF step, I"m not sure why. And yes, correct, the damped_dyanmics(n=50) ~=
"BFGS"(n=14).

However my concern is that for the "BFGS" the z component of all axis
changes, which for my use-case is very bad. However, for damp only the z
component of axis 3 (v3_z) is allowed to relax. Unfortunately, in my use
case (the one in the attached example) the damp method seems to converge
much much slower than BFGS. E.g. for the metrics that would affect my needs:

grep -A3 CELL_PARAMETERS pw.out.bfgs <fast convergence in cell shape, but
we see shifts in the z-component of axis 1 & axis 2, which for my needs is
very bad>
grep -A3 CELL_PARAMETERS pw.out.damp_pr <slow convergence in cell shape,
but we see that the z-component of axis 1 & axis 2 are held fixed, which
for my needs is very good>

Is there any way to make bfgs hold the z-component of axis 1 & 2 fixed,
like how we see for the damp_pr example?

Best,

Daniel

On Tue, Apr 30, 2019 at 2:16 PM Lorenzo Paulatto <paulatz at gmail.com> wrote:

> there are several quircks:
>
> I see you did not specify ion_dynamics="damp" when doing damped
> dynamics, I'm not sure what happens of the ions in this case.
>
> The "BFGS" output you sent me is contains 14 steps of BFGS optimization,
> followed by one step of (damped?) molecular dynamics
>
> The damped dynamics run stops because it reaches the maximum of 50
> iterations (nstep=50) not because it has converged, but the cell it
> obtains is pretty much identical to the BFGS one
>
> hth
>
> On 30/04/2019 13:55, Daniel Marchand wrote:
> > Thanks, Lorenzo, for the rapid response. I have a bit of a concern w.r.t
> > the z-only relaxation. When I use BFGS the z component of all the axis
> > are allowed to relax. However, I do not want this, I want only the z
> > component of axis 3 (v3_z) to be moved, as per the description in the
> > documentation. This only seems possible with the damp algorithm. Here I
> > have attached sample input for both the bfgs and damp_pr algorithms
> >
> https://drive.google.com/file/d/17Bt8HHL55yG75cba6awvl8E_wo20KDYI/view?usp=sharing.
>
> > Perhaps I have one of the settings wrong?
> >
> > Best,
> >
> > Daniel
> >
> >
> > On Tue, Apr 30, 2019 at 1:07 PM Lorenzo Paulatto <paulatz at gmail.com
> > <mailto:paulatz at gmail.com>> wrote:
> >
> >      > 1) Is this still the case? When I enable this setting in my own
> >      > calculations (QE v6.3) I see the cell does indeed allow only
> >      > z-relaxation in the cell, but it does so for all the lattice
> >     directions
> >      > rather than just the a_3 lattice direction.
> >
> >     In general cell_dofree works with BFGS, but it is true that some
> times
> >     damped dynamics is more reliable.
> >
> >     I.e. I recently did some relaxations with cell_dofree="shape", as
> BFGS
> >     was not findign the minimum for some volumes, I just put
> >         cell_dynamics="damp-pr"
> >         ion_dynamics = 'damp'
> >     with everything default.
> >
> >     cheers
> >
> >     --
> >     Lorenzo Paulatto - Paris
> >     _______________________________________________
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> --
> Lorenzo Paulatto - Paris
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