Re: [AMBER] Hybrid QM/MM Convergence

From: Brian Radak <>
Date: Sun, 3 Nov 2013 13:53:38 -0500

Hi Ahmed,

I think we need more details, such as output from specific minimization
steps, but two things come to mind:

1) If the SCF energy is suddenly rising, to me that means the QM
calculation is not converging and AMBER is continuing with whatever energy
and gradients it last had. This are probably NOT good energies and
gradients and will likely not give meaningful results.

2) The default minimizer in AMBER is a steepest descent/conjugate gradient
combination that is known to sometimes be both unstable and not reliably
convergent (in the sense of actually finding minima). This is generally not
an issue since this function is mostly used to remove bad contacts prior to
MD. I believe there are newer minimizers available, but you will have to
check in the manual. I don't remember the name of the option, something
like ntrmin or something?


On Sun, Nov 3, 2013 at 12:25 AM, Ahmed Ayoub <> wrote:

> Dear Amber experts,
> I'm doing a hybrid QM/MM minimization using Gaussian 09 as an external QM
> agent. When I specify the number of minimization steps in the CTRL
> namelist, I sometimes notice a bizarre behaviour. The minimizer keeps
> minimizing the SCF energy and at one point of time, the SCF energy rises up
> again, sometimes tremendously, and again the minimzer restarts minimizing.
> This keeps going till the max number of cycles is reached which could yield
> an unminimized structure. This is kind of not what I want. I want basically
> to reach convergence on the energy of the QM region. Is there any way to
> avoid this behavior, or to specify a certain gradient for convergence
> instead of using the maximum cycle approach?
> Any help would be appreciated.
> Ahmed
> _______________________________________________
> AMBER mailing list

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 Brian Radak                                             :     BioMaPS
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Received on Sun Nov 03 2013 - 11:00:02 PST
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