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

Cristiana Passiu uido... at
Wed May 10 11:42:44 UTC 2017

Dear Vladimir,
thank you for your answer. It will be helpful for knowing how to express 
the results of the calculations.

Best regards,

Il giorno mercoledì 10 maggio 2017 10:07:57 UTC+2, Vladimir Rybkin ha 
> 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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