This hadn't really occurred to me until an experimentalist with no experience in computational chemistry asked me "to how many significant figures is Gaussian accurate when measuring distance?"
I think Gaussian's internal algorithms compute in Bohr radius but outputs in angstroms. In GView until now I was using the exact numeric-figure when measuring distance., e.g., 0.95708 angstroms, which is 1.8086190 Bohr radius. However, (I am not a physicist) what is 0.8, 0.00, 0.008, 0.0006, 0.00001 etc..., of a Bohr radius? Do these significant figures make any sense? Therefore, how many significant figures in an angstrom make sense.
I fear that this question might get a bit philosophical. However, I just wish to know to what signifiant figure ought I publish using distances derived in Gaussian.
Thanks
Anthony
I think you have two questions:
1) How precise are the convergence algorithms used for the wavefunction and geometry searching. These tolerances give you an idea of the precision of the specific calculation.
2) How accurate is the method used. (If the method has a known error of say 0.01 A for a certain bond length, there doesn't seem to be much point in converging the geometry to 0.0000001 A)
It depends on the method and the convergence criteria you use. Some of this info is available in the output file.
Many thanks Bradley, I'll dig deeper in the output files. See what I can find.
We must differentiate between numerical precision (Bradley's 1)) and physical precision (comparable with experiment). I can add to Bradley, that in the last case it is also dependent on considered system.
With respects to optimisation method it would have been Berny using GEDIIS/GDIIS optimiser, set to a tight convergence criteria (int=ultrafine for DFT) and the SCF would be default (which is tight also).
Dear Anthony,
Generally for bond distance is 0.001 A; for energy 0.1 Kcal/mol.
I got these numbers from tables that compare values calculated by different method.
See attached file.
Best wishes,
Rafik
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