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29 votes
Accepted

How accurate are the most accurate calculations?

Exhibit 1: Ground state hyperfine splitting of the H atom: ...
Nike Dattani - No Free Time's user avatar
27 votes
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Is it right to neglect very small imaginary frequencies?

tldr: This is something of an eternal debate. IMHO very small imaginary frequencies can be okay, but it depends on your system and needs. As you might see from the various comments above, there are ...
Geoff Hutchison's user avatar
21 votes
Accepted

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

The paper in your question currently has 159 citations and was published in one of the most prestigious journals (Physical Review Letters). It is about high-precision spectoscopy on $\ce{H2}$, which ...
Nike Dattani - No Free Time's user avatar
18 votes

Is it right to neglect very small imaginary frequencies?

Just want to add a small comment upon Geoff's excellent answer. One may be tempted to think that a $<\pu{10 cm-1}$ imaginary frequency introduces a negligible error to the energy or the property of ...
wzkchem5's user avatar
  • 9,670
12 votes
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Are there examples of ab initio predictions on small molecules without the "major approximations"?

I wrote an answer to a similar question in the past, but focused in that question only on the state-of-the-art ultra-high precision calculations on atoms and the three most common isotopologues of $\...
Nike Dattani - No Free Time's user avatar
12 votes
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High precision helium energy

The 2006 variational calculation by Schwartz is lower (more accurate) than Nakashima & Nakatsuji's 2007 energy: ...
Nike Dattani - No Free Time's user avatar
11 votes

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

One of the eternal questions in computational modeling is whether we truly understand all the relevant contributions. I believe the combination of experiment and theory in the paper suggest that if ...
Geoff Hutchison's user avatar
11 votes
Accepted

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

First of all, MP2 (for example) is actually guaranteed to converge from above to the basis set limit even though MP2 in one specific basis set can give an energy lower than the FCI energy in that same ...
Nike Dattani - No Free Time's user avatar
10 votes

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

In general, when computing any property with different models (e.g. level of theory, basis set, etc), if you don't have some kind of theoretical bound (like the variational principle) to determine ...
Tyberius's user avatar
  • 15.5k
8 votes

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

Your arguments can be said about numerous other articles (if not all but a very few ones). What seems negligible in scientific impact at the publication time might be heavily influenced by other ...
nickpapior's user avatar
  • 3,494
6 votes
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How much does it cost (RAM and CPU) to calculate the energy of H2?

Out of all authors on that paper, most of them are experimentalists, but Krzysztof Pachucki is known as the "King of H2 calculations". Three years after the paper you mentioned, Pachucki ...
Nike Dattani - No Free Time's user avatar
5 votes
Accepted

Is there a standard file format for storing one and two electron integrals?

The FCIDUMP format is probably the closest thing to what you are seeking. More than a dozen different quantum chemistry programs already have a built-in mechanism ...
Nike Dattani - No Free Time's user avatar
5 votes
Accepted

High accuracy energy methods, are they worth it?

High accuracy energy methods, are they worth it? They are absolutely worth it for many types of studies, but not worth it for other types of studies. It depends on precisely what the aim of your ...
Nike Dattani - No Free Time's user avatar
5 votes

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

Well, Nike already answered the point about the variationality: even though methods like MP2, CCSD, and CCSD(T) are non-variational in that they may over- or underestimate the energy of the ground ...
Susi Lehtola's user avatar
  • 19.7k
5 votes

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

If you want to test how good your theory is, you need a precise calculation produced using that theory, and you need a precise measurement to compare it against. Spectroscopy is generally the best way ...
llama's user avatar
  • 151
5 votes

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

What's the point of doing high accuracy spectroscopy calculations? I think there's a more fundamental answer. But I'm going to argue from the experimental side. Of course implicit in my answer is the ...
uhoh's user avatar
  • 2,034
2 votes

Which programs can calculate corrections to the Born-Oppenheimer approximation?

CFOUR + MRCC You asked about MOLPRO and Psi4. As recently as 2021, DBOC (diagonal Born-Oppenheimer correction) wasn't implemented in Psi4. The same is true for MOLPRO. No other types of Born-...
Nike Dattani - No Free Time's user avatar
1 vote

Why is my CISD restart failing in PySCF?

To prioritize other people's questions in search results for unanswered questions, I'm taking this out of the unanswered queue. If have also promised to put a bounty on this question if someone does ...
1 vote

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

Advantage for QMC: While this question is quite open ended due to there being at least 15 different types of QMC, one thing that all 15 of those methods have in common is that as far as I know they ...
Nike Dattani - No Free Time's user avatar

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