# Tag Info

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 ...
• 1,021
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) ...
• 1,053

### 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 ...
• 808

### 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 ...
• 15.1k
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 ...
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) ...
• 4,382
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 ...
• 29.2k

### 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 ...
• 7,478

### 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 ...
• 14.3k
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### 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 ...
• 6,941
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 ...
• 29.2k
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### 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 ...
• 1,021

### 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 ...
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 ...
• 6,941
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### 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 ...
• 15.1k
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### 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 ...
• 4,382
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-...
• 2,314
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 ...
• 14.3k

### 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 ...
• 856
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### 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 ...
• 29.2k
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### 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 (...
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### 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 ...
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### 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 ...
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### 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 ...
• 14.3k
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### 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&...
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### 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 ...
• 2,561
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 ...
• 4,382

### 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-...
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### How is "basis set projection" done?

What you're describing is very common, and is not limited to GAMESS and Q-Chem. First, here's how to do it in MOLPRO, MRCC, GAMESS, Q-Chem (in fact the only electronic structure software that I ...
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Your formula for the analytical $\ce{H}$ 1S orbital is incorrect. A 1S orbital has the form $$\phi_{1s}(\zeta_1,r)=\sqrt{\frac{\zeta_1^3}{\pi}}\exp(-\zeta_1 r)$$ For $\ce{H}$ the optimal value for \$\...