How can I estimate the uncertainty on a calculated energy value?

[Cristiana Passiu]

Dear Filipe,
thank you for your reply and for the interesting paper suggested.

Kind regards,
Cristiana


Il giorno giovedì 11 maggio 2017 11:59:30 UTC+2, Filipe Teixeira ha scritto:
>
> Hello
>
> In the early 2000's Irikura introduced the concept of "virtual 
> meassurement" (J. Phys. Chem. A, 2005, 109, 8430). In this article one of 
> the sources of error he explored is the error associated with the SCF of 
> optimied geometries (which may depend, for example on the initial geometry 
> being optimized and the propagation of numerical errors in the SCF 
> procedures). It is an interesting reading if you are interested on that 
> matter, but (spoiller allert) those errors (even in 2005 when 32-bit 
> machines were common) are minor compared to the errors of the methodologies 
> themselves. Nowadays, with 64-bit machines being the norm, I doubt machine 
> round-up will be a big source of error for most applications... 
>
> I hope this helps.
>
> Best regards,
> Filipe Teixeira
>
> On Wednesday, May 10, 2017 at 12:42:45 PM UTC+1, Cristiana Passiu wrote:
>>
>> Dear Vladimir,
>> thank you for your answer. It will be helpful for knowing how to express 
>> the results of the calculations.
>>
>> Best regards,
>> Cristiana
>>
>>
>>
>>
>> Il giorno mercoledì 10 maggio 2017 10:07:57 UTC+2, Vladimir Rybkin ha 
>> scritto:
>>>
>>> Dear Cristiana,
>>>
>>> the question has no correct answer. To give a few points:
>>> 1) the energy here is in Hartree (that's the only thing for sure here)
>>> 2) the energy itself is meaningless, what is reported is the energy 
>>> difference. In kJ/mol people report them with one digit, which, however 
>>> does not mean that the uncertainty is 0.05. It's just a convention.
>>> 3) try to find benchmarks on your systems and use the recommended 
>>> methods with estimated accuracy.
>>>
>>> Yours,
>>>
>>> Vladimir
>>>
>>> понедельник, 24 апреля 2017 г., 10:55:29 UTC+2 пользователь Cristiana 
>>> Passiu написал:
>>>>
>>>> Dear all,
>>>> assuming that a model is correct and that the functional chosen is 
>>>> suitable for the system, is there a standard uncertainty that can be 
>>>> attributed to the final energy values? In other words, how many 
>>>> significative digits can I report in the results?
>>>>
>>>> What is the default unit of measurement for the energy in the output? 
>>>> For example, if I have this in the output:
>>>>
>>>>   Total energy:                                             
>>>>  -158.42425955059213
>>>>  ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):             
>>>> -158.424259550592126
>>>>
>>>> are the values in Rydberg or in Hartree? In order to convert them to 
>>>> eV, do I have to multiply them by 27.2113838668?
>>>>
>>>> Many thanks,
>>>> Cristiana
>>>>
>>>>
>>>>
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