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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3 votes
1 answer

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 ...
4 votes
1 answer

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

I need to calculate the electric dipole moment of molecules like HD, in which the electric dipole moment is zero in the Born-Oppenheimer approximation (meaning that it's absolutely necessary to ...
8 votes
1 answer

How much does it cost (RAM and CPU) to calculate the energy of H2?

I have recently come across this very intriguing paper “The Fundamental Vibration of Molecular Hydrogen”. I have asked the authors the following naïve questions directly via email: What was the ...
15 votes
5 answers

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 ...
27 votes
2 answers

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 ...
9 votes
1 answer

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

Is there any standard file format (.txt, .dat etc) for storing one and two electron integrals (generated from ab initio packages) which doesn't compromise on precision and is convenient for reading ...
7 votes
1 answer

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 ...
28 votes
1 answer

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 ...
7 votes
1 answer

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 ...
15 votes
3 answers

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, ...
12 votes
1 answer

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
1 answer

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....