Re: [AMBER] errors in qm/mm

From: Andreas Goetz <>
Date: Sun, 23 Nov 2014 22:27:33 -0800

Dear Ayesha,

You are trying to converge the SCF very tightly. Probably your electron density is well converged but not to the thresholds you requested. This might be the reason for failure. Try setting tight_p_conv=0 (this is default) and possibly scfconv=1.0e-8 which should be sufficient.

I also noticed you are using a very large cutoff. Using cut=9.0 is usually sufficient.

All the best,

Dr. Andreas W. Goetz
Assistant Project Scientist
San Diego Supercomputer Center
Tel  : +1-858-822-4771
Web  :
On Nov 23, 2014, at 10:10 PM, Ayesha Fatima wrote:
> Dear all,
> I am trying to run a qm/mm on a protein-ligand system. I have successfully
> done classical MD without errors. As per information on the mailing list,
> for qm/mm, i used a well equilibrated .rst file from the classical MM
> method and was able to do further equilibration with qm/mm using the PM3
> Hamiltonian. It is in the MD production phase.
> I want to repeat the same with SCC-DFTB but unfortunately it keeps giving
> errors in the equilibration phase such as
> QMMM SCC-DFTB: !!!! ============= WARNING ============= !!!!
> QMMM SCC-DFTB: Convergence could not be achieved in this step.
> QMMM SCC-DFTB: The calculation will continue, but energies and
> QMMM SCC-DFTB: forces for this step will not be accurate.
> I have looked around on the mailing list and know its a common error. I
> also tried using the PM3 equilibrated file and trying to further
> equilibrate it with the SCC-DFTB. But the error still remains. I am
> attaching my input file. I am sure someone has done it and resolved the
> issue. Could they share their experience?
> Thank you
> Regards
> Ayesha Fatima
> PhD candidate UM
> <>_______________________________________________
> AMBER mailing list
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Received on Sun Nov 23 2014 - 22:30:03 PST
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