36 votes
Accepted

Approximate equivalence table between Pople's basis sets and Jensen's DFT optimized polarization consistent basis sets (pcseg-n)

Disclaimer and warning: long and likely biased answer. Background: The Pople style basis sets were defined almost 50 years ago. The 6-31G was designed for HF calculations, the 6-311G for MP2 ...
Frank Jensen's user avatar
  • 1,726
17 votes

What are the positives and negatives of periodic DFT codes that don't use plane-wave basis sets?

The main positives: you can do all-electron calculations you don't need to set up pseudopotentials / PAWs you can study core properties you can use hybrid functionals cheaper / run post-HF ...
Susi Lehtola's user avatar
  • 18.4k
17 votes
Accepted

What functions other than Gaussians are used for orbital basis sets?

There are two considerations guiding the choice a basis for orbital expansion: 1. Compactness; 2. Efficiency of computations. There are two common choices for basis functions (A) Gaussians and (B) ...
Roman Korol's user avatar
  • 1,073
16 votes

What functions other than Gaussians are used for orbital basis sets?

I would like to expand on Roman Korol's answer a bit. He already lists GTO's and plane waves as they are the most common kind of basis functions. These are characteristic for the underlying models by ...
Fuzzy's user avatar
  • 928
16 votes
Accepted

What are the positives and negatives of periodic DFT codes that don't use plane-wave basis sets?

Pure plane-wave basis sets have the following advantages when used in periodic DFT (or HF) simulations: Orthogonal Computationally simple (operators with derivatives are particularly straightforward) ...
Phil Hasnip's user avatar
  • 7,022
15 votes
Accepted

What are the scientific justifications of the binding energy equation?

The justification is simple and comes from a very fundamental law of thermodynamics: Internal energy is a complete differential form and is independent of intermediate states and only depends on start ...
Mithridates the Great's user avatar
15 votes
Accepted

Gaussian-type orbitals vs Slater-type orbitals in terms of computational efficiency

Your supervisor is correct that in almost all practical cases in quantum chemistry, the cost savings of using Gaussian-type orbitals far outweigh the disadvantages of needing more orbitals. First of ...
Nike Dattani's user avatar
  • 34.7k
14 votes

What are the positives and negatives of periodic DFT codes that don't use plane-wave basis sets?

One important property of atom-centered basis sets is that electrons can only be localized on atoms. This is a problematic property when modeling solid systems with defects. For instance, at a color ...
Franziska Heß's user avatar
14 votes
Accepted

What methods can be used to calculate excited-states in molecular crystals?

There's two facets to this question: What methods can be used for excited states in crystals? (the title, and final sentence) What methods can be used for excited states with plane waves? (paragraphs ...
Nike Dattani's user avatar
  • 34.7k
14 votes

What is the minimum basis set one should use?

Different people will have slightly different opinions, so take my answer with a grain of salt. I'll try to give the minimum publishable basis sets, i.e. if you use worse basis sets than these, you'll ...
wzkchem5's user avatar
  • 9,376
14 votes
Accepted

Basis sets optimized for CCSDT(Q) calculations

To the best of my knowledge, there have not been basis sets optimized for post-CCSD(T) calculations. Note that the cc-pVXZ have been optimized using CISD. One would expect these to be also close to ...
Frank Jensen's user avatar
  • 1,726
13 votes
Accepted

Are plane-wave basis sets reliable for modeling adsorption processes?

In the example you highlighted and indeed in most plane-wave DFT codes, there is periodicity in all three dimensions including for surface slab calculations. In the case of a surface slab, vacuum ...
Andrew Rosen's user avatar
  • 7,301
13 votes

What functions other than Gaussians are used for orbital basis sets?

Another slightly less commonly seen basis are the sinc functions, which are related to plane waves, but come at the problem from the perspective of a position, rather than momentum, space. They are ...
Tyberius's user avatar
  • 15.4k
12 votes
Accepted

Why were no correlation consistent basis sets for potassium contracted for a non-relativistic Hamiltonian?

The answer in part is that not all the elements up through $\ce{Xe}$ have (or had until fairly recently) a nonrelativistic correlation consistent basis set. Specifically, the alkali and alkaline earth ...
Tyberius's user avatar
  • 15.4k
12 votes
Accepted

When are atomic-orbital-basis (rather than plane-wave) methods appropriate in periodic DFT?

The clearest example in my mind is if you want to understand the orbital-based contributions to some phenomena (e.g. bonding, a reaction energy), particularly if the periodic material being modeled is ...
Andrew Rosen's user avatar
  • 7,301
12 votes
Accepted

Which basis set should be used for a system that contains both anions and cations?

The safe choice is to use diffuse functions on all atoms. It is quite rare to run into pathological overcompleteness with standard augmented basis sets, unless you're looking at very high-energy ...
Susi Lehtola's user avatar
  • 18.4k
12 votes
Accepted

Optimization of Gaussian basis sets within the Hartree-Fock Method

There are a lot of different methods, but a good starting point to understand how this might be done is the method of steepest descent. First we take the generalised eigenvalue expression you cite and ...
Phil Hasnip's user avatar
  • 7,022
11 votes
Accepted

How does a complete basis set remove Pulay forces/stress?

I think a good way to see this is to simplify the stage a little. Imagine we want to solve the Schrödinger equation for a Hamiltonian $H$. For this we take a very simple basis, namely just two real-...
Michael F. Herbst's user avatar
11 votes

What functions other than Gaussians are used for orbital basis sets?

The question was about "orbital basis sets" but explicitly mentions Gaussians; I guess the topic here is atomic orbital basis sets. In this case, the molecular orbitals are expanded as a ...
Susi Lehtola's user avatar
  • 18.4k
11 votes

What are the advantages of plane wave basis?

In my opinion a solid, brief overview of DFT is given here. https://www.archer.ac.uk/training/course-material/2014/04/PMMP_UCL/Slides/castep_1.pdf The pro/cons of plane waves and other basis sets ...
CKl's user avatar
  • 876
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's user avatar
  • 34.7k
10 votes
Accepted

Predicting linear dependencies in a basis set in advance of calculating any integrals

You might be interested to know that I've recently presented a general solution to this problem in Curing basis set overcompleteness with pivoted Cholesky decompositions, J. Chem. Phys. 151, 241102 (...
Susi Lehtola's user avatar
  • 18.4k
10 votes
Accepted

How to interpret molden AO/MO coefficients?

Okay, so there are many layers to this question. cc-pVTZ for H is [5s2p1d/3s], which comes out to 3 + 2*3 + 5 = 3+6+5 = 14 basis functions per atom, which are composed of 16 primitives (the contracted ...
Susi Lehtola's user avatar
  • 18.4k
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
  • 15.4k
10 votes
Accepted

Molecular orbitals and active space

As you said above, nitrogen has 2 s orbitals and one p orbital. However, one would typically freeze the chemically inactive 1s orbital, leaving you 5 electrons in 4 orbitals per atom, or 10 electrons ...
Susi Lehtola's user avatar
  • 18.4k
10 votes
Accepted

How can I find the parameters in a basis set?

Go to Basis Set Exchange and click on the elements in which you're interested, then click the basis set in which you're interested (in your question, you mentioned 6-31G) and then for "Format&...
Nike Dattani's user avatar
  • 34.7k
9 votes

What are the scientific justifications of the binding energy equation?

In general, a system composed of $K$ interacting subsystems have a potential energy at a specific configuration of its parts. For instance, a system of $M$ nuclei and $N$ electrons can be separated ...
Verktaj's user avatar
  • 2,701
9 votes
Accepted

How trustworthy is the Tran-Blaha modified Becke-Johnson potential and how does it perform across basis sets?

The Tran-Blaha functional is not a generally useful functional, so I would not describe it as "trustworthy" at all. For example, it is not size-extensive so twice as much material does not ...
Phil Hasnip's user avatar
  • 7,022
9 votes

What's the best way to compare two DFT codes?

There are many options to answer your question. Just to compare the computation time, the first condition is to run the simulations with the same hardware i.e. in the same system (two different set-...
Camps's user avatar
  • 22.9k
9 votes
Accepted

Is basis set superposition error reduced when using the GAPW method?

Short answer: no. The idea of the GAPW method described in Theor. Chem. Acc. 103, 124 (1999) is simply to speed up the evaluation of the Coulomb and exchange-correlation contributions. Quoting from ...
Susi Lehtola's user avatar
  • 18.4k

Only top scored, non community-wiki answers of a minimum length are eligible