Re: [AMBER] QM/MM energy terms

From: Andreas Goetz <>
Date: Sun, 7 Nov 2010 16:56:59 -0800

Dear Davide,

Here is information on the energy terms:

DFTBESCF should be comparable to a DFTB calculation with any other software package if you include surrounding MM atom point charges as external point charges in the calculation.

A couple of comments:
*) If you use external point charges in Gaussian (or any other QM package) you won't get the vdW interactions between QM and MM system.
*) Be aware that only relative energies are of relevance, comparison of single point energies between different methods does not make sense.

All the best,

On Nov 7, 2010, at 4:04 PM, Davide Branduardi wrote:

> Dear Amber Users,
> I'm performing some QM/MM calculations with DFTB and now I would like to check
> how far DFTB energies are compared to some typical DFT functonal (say B3LYP).
> So, for a set of snapshots I extracted the QM system (link atoms included) and the sourrounding
> charges and I used them to calculate the energy with G09 by explicitely introducing
> the point charges via the CHARGE keyword. From the g09 output I extracted the external point charge energy from
> the SCF energy and now I think I am ready to compare the QM+QM/MM energy from sander
> and the QM/Point charges energy in Gaussian.
> However at this stage I am a bit confused.
> Which is the best term to compare with in sander output?
> I would say that the DFTBESCF term is what I need, right?
> Is this term including the QM+QM/MM term or does it contain the QM part alone ?
> Do you have any recommendation on how to perform this task?
> Thank you for your help.
> Have a nice weekend
> Davide
> **************************************************************
> Davide Branduardi
> Post Doc- Computational Chemistry
> Drug Discovery and Development
> Via Morego 30, 16163 - Genova, Italy
> Phone: +39 010 71781573 Room: 1/27
> Fax: +39 010 7170187
> e-mail:
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Dr. Andreas W. Goetz
San Diego Supercomputer Center
Tel  : +1-858-822-4771
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Received on Sun Nov 07 2010 - 17:00:04 PST
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