Re: [AMBER] QM/MM energy terms with DFTB

From: Ross Walker <>
Date: Thu, 16 Dec 2010 14:17:35 -0800

Hi Markus,

> Question: What is included in the term 'DFTBESCF'
> Is it only the QM energy of the QM region (terminated with H atoms)
> and electrostatic interaction of QM nuclei and electron cloud with
> unscaled point charges of the MM region?
> Vdw interaction between QM and MM region is in included into 'VDWAALS'
> and '1-4 VDW' ?
> Possible bond, angle and dihedral terms between QM and MM regions are
> included into the classical terms (BOND, ANGLE, DIHED)?
> Solvation energy of QM region is in included into EGB ?

Yes you are correct with some potential caveats. Essentially you have this
for GB calculations:

BOND = All classical bonds excluding anything QM-QM bonds (It DOES include
QM-MM bonds classically)
ANGLE = All classical angles excluding anything involving pure QM atoms.
(QM-QM-MM and QM-MM-MM angles are included classically)
DIHED = All classical dihedrals excluding anything involving pure QM atoms.
(QM-QM-QM-MM and QM-QM- MM-MM and QM-MM-MM-MM dihedrals are included
VDWAALS = All classical VDW excluding QM-QM interactions and 1-2,1-3 and 1-4
exclusions. Note QM-MM interactions are included here classically. QM link
atoms are excluded.
1-4 VDW = All classical 1-4 VDW interactions, scaled by SCNB (typically
2.0). All 1-4 VDW interactions are included unless ALL atoms are QM.
EEL = All classical MM-MM point charge interactions in the direct space.
1-4 EEL = All classical 1-4 EEL interactions scaled by SCEE (typically 1.2).
Interactions involving one or more QM atoms are excluded.
EGB = All classical MM solvation terms due to the use of GB. Also included
here is the QM GB solvation term due to the mulliken charges on the QM atoms
calculated within the GB field. QM link atoms also feature here. The MM
atoms bound directly to the QM region (which are replaced by link atoms) are
DFTBESCF = This is the tricky part. This includes the QM energy but also the
QM-MM interaction energy. It also includes 'part' of the solvation energy.
The reason for this is that the GB field is included in the Fock matrix
during the SCF. This results in the QM density matrix being different from
the 'gas phase' density which essentially that the QM regions response to
the GB solvation is included as an energy in the DFTBESCF term. As far as I
know there is no 'simple' way to back this back out and decompose it. One
could potentially recover it by repeating calculations in gas phase for the
same structures and comparing the difference.

I hope this makes sense.

All the best

|\oss Walker

| Assistant Research Professor |
| San Diego Supercomputer Center |
| Adjunct Assistant Professor |
| Dept. of Chemistry and Biochemistry |
| University of California San Diego |
| NVIDIA Fellow |
| | |
| Tel: +1 858 822 0854 | EMail:- |

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Received on Thu Dec 16 2010 - 14:30:02 PST
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