Questions tagged [basis-set]

A set of (basis) functions make up a basis set. These functions (generally) represent atomic orbitals whose linear combinations are used to make molecular orbitals (LCAO-MO) in computational chemistry.

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Computing two-electron integrals with an STO-3G basis set

I am trying to implement a restricted Hartree-Fock calculation using an STO-3G basis set, for fun. I managed to perform this calculation where only $\mathrm{1s}$ orbitals are present ($\ce{H2}$ and $\...
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2answers
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Equivalent of Szabo and Ostlund book for DFT

Szabo and Ostlund book Modern quantum chemistry [1] is extremely useful to understand Hartree-Fock and post-Hartree-Fock methods. Not only it explains the theory behind such methods, but it is also ...
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2answers
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Why are correlation consistent basis sets used with DFT calculations?

Reading some papers, there is a bit of a curious observation that does not exactly make a lot of sense to me. In Quantum Chemistry, different methods exist to carry out geometry optimisation and ...
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3answers
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Why are basis sets needed?

I am not sure whether this question is even reasonable, but here it goes. We are taught about the different types of basis sets (extended, minimal, double-zeta, plane wave), but I do not think it is ...
16
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2answers
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How to compute 2-electron integral for Hartree-Fock code?

I'm working on writing my own code for $\ce{H2}$ in an STO-3G basis set using Hartree-Fock (HF), and I am currently stuck on how to construct the two-electron integral matrix. I know how to evaluate ...
14
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1answer
5k views

How many basis functions used in STO-3G and 6-31+G** for the water molecule?

I am a little confused about how to decide how many basis functions that are used in a particular basis set for a given molecule. STO-3G is a so-called "minimal basis set", meaning that only one ...
15
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3answers
3k views

Boys function for Gaussian integrals in ab-initio calculations

A few days ago I mentioned a problem with an Hartree-Fock program I am writing (HF using cartesian Pople's STO-3G basis set). I can reproduce overlap and kinetic integrals of some references for ($\ce{...
8
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1answer
418 views

How does one compute the number of unique 2-electron integrals for a given basis set?

I have run into trouble with an exercise in Szabo and Ostlund's Modern Quantum Chemistry. Exercise 3.14: Assume the basis functions are real and use the symmetry of the two-electron integrals [$(\...
15
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1answer
997 views

Naming of quantum chemistry basis sets

For some DFT calculations I performed with localized (Gaussian) basis sets, I wonder how the basis sets I used are properly named. The names I could gather are: H, C and O: 6-311G(d,p) Al: 8-511G* ...