[gmx-developers] double precision minimization accuracy limits.

[Erik Lindahl]

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