Questions tagged [high-precision]

Questions about high-precision or high-accuracy calculations (even though these two things don't meant he same thing, they are related enough to fit in one tag for now).

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28 votes
1 answer
650 views

How accurate are the most accurate calculations?

Taking into account the fact that the theory of quantum gravity does not exist and the QED calculations are not possible for most realistic chemical systems, what levels of accuracy can we expect from ...
Roman Korol's user avatar
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12 votes
1 answer
494 views

Are there examples of ab initio predictions on small molecules without the "major approximations"?

In most textbooks the precise Schrödinger euqation of a molecule is given (and the "beautiful thing" stops here), then born oppenheimer approximation is made, then layers of other ...
jw_'s user avatar
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27 votes
2 answers
3k views

Is it right to neglect very small imaginary frequencies?

I'm running a DFT optimization (B3LYP/def2-TZVP) and frequencies in ORCA for a molecule. And I get one or two very small (1-6 cm^-1) imaginary frequencies which corresponds to a slight bend of the ...
Roman's user avatar
  • 2,373
9 votes
1 answer
506 views

High precision helium energy

In the paper "High Precision Theory of Atomic Helium", Drake lists the then best-known ground state energy of the Schrödinger equation for helium to 22 digits as: $$\lambda_0 \approx -2....
user14717's user avatar
  • 359
3 votes
1 answer
99 views

Why is my CISD restart failing in PySCF?

A case that works In my answer to Restarting an FCI calculation in PySCF, I provided a folder full of input and output files that demonstrates saving (as a numpy ...
I have no free time anymore's user avatar
15 votes
5 answers
2k views

What's the point of doing high accuracy spectroscopy calculations?

I am currently reading Dickenson et al. (2013) The Fundamental Vibration of Molecular Hydrogen. I am trying to understand why this calculation is important for the field beyond basic scientific ...
Omar Shehab's user avatar