[AMBER] QM/MM using Amber and Gaussian: Discrepancies between the QM energy from the gaussian output file and the energy reported in the mdout file as EXTERNESCF

From: Eric Lang <eric.lang.bristol.ac.uk>
Date: Tue, 4 Dec 2018 11:36:20 +0000

Dear Amber Developers and Users,

I am running some single point QM/MM in Sander (compiled as Amber16+AmberTools17) using Gaussian 09.

The calculations run ok, but I think something is not quite right with the extraction of the QM energy from the gaussian output file: the EXTERNESCF energy in the mdout file in kcal/mol doesn’t match the final energy (last ‘SCF Done’ line or ‘HF’ at the very end of the Gaussian file) when converted to kcal/mol using the conversion used in Amber (AU_TO_KCAL = 6.2750946943E+02).

I would think they should be the same.

Am I missing something obvious in the way the calculations are done or is there a problem with the way the gaussian output files are parsed?

Here is the output form Amber:

                    FINAL RESULTS


      1 -5.0519E+05 1.3953E+01 1.1652E+02 CA 1579

 BOND = 456.4977 ANGLE = 1327.2559 DIHED = 2177.6750

 VDWAALS = 8267.2236 EEL = -89612.1756 HBOND = 0.0000

 1-4 VDW = 625.9136 1-4 EEL = 7948.6914 RESTRAINT = 0.0000

 EXTERNESCF = -436379.7510

And here is the final energy from Gaussian

SCF Done: E(RwB97XD) = -3178.95512525 A.U.

Which gives -1994824.4 kcal/mol and not -436379.7510 kcal/mol as found in EXTERNESCF.

Many thanks in advance for your help,


Eric Lang
Centre for Computational Chemistry
School of Chemistry - University of Bristol
Bristol BS8 1TS - United Kingdom
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Received on Tue Dec 04 2018 - 04:00:03 PST
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