# Tag Info

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### Why are correlation consistent basis sets used with DFT calculations?

It is generally recommended not to use a cc basis set with a DFT method (and I guess conversely, a basis set aimed at DFT should not be used with a coupled cluster method). This statement glosses ...
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### Why are basis sets needed?

Spatial orbitals $\phi_i$ in modern electronic structure calculations are indeed typically expressed as a linear combination of a finite number of basis functions $\chi_k$, \begin{equation} \phi_i(...
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### "NBO diagrams" versus MO diagrams

A molecular orbital diagram is a schematic representation of how we interpret bonding in certain species. It is as much an accurate representation for a specific bonding situation as a Lewis structure ...

### Matching a Slater-type wavefunction with a minimal (STO-nG) Gaussian basis set

TL;DR The procedure to represent a Slater-type orbital (STO) as a linear combination of Gaussian-type orbitals (GTO) is outlined in W. J. Hehre, R. F. Stewart and J. A. Pople, J. Chem. Phys. 1969, 51,...
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### How to compute 2-electron integral for Hartree-Fock code?

You can also write this down in a similar way as for the one-electron integral. You already had: \begin{equation} \mathbf{S} = \mathbf{D_2}' * \mathbf{S}_{\rm prim} * \mathbf{D_2} \end{equation} ...
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### How many basis functions used in STO-3G and 6-31+G** for the water molecule?

Notation ** is just a synonym for (d,p), so that 6-31+G** basis is just a different name of ...
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### Computing two-electron integrals with an STO-3G basis set

Actually there is a mistake in the analtical expression in Cook's Book. On his web page he has a pdf with the corrected verison http://spider.shef.ac.uk/ Maybe this solves your problem, but I would ...
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### How does one compute the number of unique 2-electron integrals for a given basis set?

To find the number of unique $2e^-$ integrals $\left<AB|CD\right>$, it is useful to first find the number of unique $1e^-$ integrals $\left<A|B\right>$. If $n$ is the number of real ...
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### How do the def2-SV, def2-SV(P), and def2-SVP basis sets differ?

Def2-SVP has the polarisation functions on all atoms, -SV(P) does not have these functions on the hydrogen atoms and -SV does without them. In future, you can check on the EMSL Basis Set Exchange It ...