[gmx-developers] double precision minimization accuracy limits.
Erik Lindahl
lindahl at cbr.su.se
Wed Jan 23 10:48:29 CET 2008
Hi,
On Jan 23, 2008, at 9:59 AM, Berk Hess wrote:
>>
> Water is a very difficult case, since the energy landscape is
> extremely anharmonic.
> I can get to a force of 0.01 though with lbfgs, cg can also get
> there, but takes much longer.
> For cg I had to make the initial step size bigger (!), em_step=0.1,
> otherwise is stopped at the first step.
>
> You did not report you cut-off settings, but for such accuracy, you
> need to make sure that the buffer
> of your switch potentials (or better still, shift) is large enough
> to accomodate twice the charge group
> radii, for water a 0.15 nm buffer is large enough.
For l-bfgs it can help convergence a lot to use switch/shift with
sufficiently large buffers, since the algorithms works by creating an
iterative approximation to the inverse Hessian. Without switch/shift
the Hamiltonian of the system is a moving target...
Cheers,
Erik
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