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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26
votes
1answer
410 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 ...
15
votes
3answers
319 views

How should I compare different basis sets for post-Hartree-Fock methods?

HF or DFT methods are variational so I can confidently say that any basis set that gives the lowest energy is the best for that system. However, I have learnt that post-HF methods such as MP2, MP3, ...
15
votes
1answer
234 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 ...
12
votes
1answer
404 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 ...
9
votes
1answer
393 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....
7
votes
1answer
75 views

High accuracy energy methods, are they worth it?

I'm trying to figure out for myself the necessity of high-accuracy compound methods for energy like W1 or Gn families. I'm learning computational chemistry by myself and sometimes lack systematical ...
7
votes
1answer
92 views

What are the advantages/disadvantages of QMC over ACFDT-RPA?

I had some experience with the ACFDT-RPA, but despite its promising features it's not practical most of the time, and for that reason, I'm looking for a new method for reference calculations. QMC is ...