Re: [AMBER] Hybrid QM/MM Convergence

From: Tanmoy Paul <>
Date: Fri, 8 Nov 2013 18:58:17 +0530

Dear Ahmed,
                   If you are dealing with large molecules then I will
recommend you to
 minimise your system using MM first . Because sometime the crystal
 itself contains some guess part. Thus a MM minimised structure will give
you a
 better structure to start with .Hope this helps


On Sun, Nov 3, 2013 at 9:55 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
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Received on Fri Nov 08 2013 - 05:30:03 PST
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