Questions tagged [electron-correlation]

Questions relating to electron correlation - the interaction between electrons in the electronic structure of a quantum system. The correlation energy is a measure of how much the movement of one electron is influenced by the presence of all other electrons.

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9 votes
2 answers
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What is the physical meaning of the term "non-local interaction"?

This term pops up through out Hartree-Fock and DFT. Does this term have any physical significance to actual(real) system or it is a mathematical by-product?
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5 votes
1 answer
65 views

Are multiplet free atoms multireference?

In general, my understanding is that free radicals and non-equilibrium molecules sometimes need multireference electronic structure calculations, mainly because of different electronic states being ...
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7 votes
2 answers
177 views

Convert ase atoms to pymatgen structure

I have atom_row from ase_db. I have tried various methods but they all fail. Seems like I'm missing a lattice, but I don't know what that is and where I can get it ...
17 votes
1 answer
883 views

What is static correlation, actually?

My question is inspired by this question and also this one. I have come across the term static correlation multiple times. Introduction to Computational Chemistry by Frank Jensen describes static (or ...
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14 votes
1 answer
235 views

Why are Norm-conserving Pseudopotentials considered so important?

So I am aware that a norm-conserving pseudopotential enforces the conditions that: Inside the cut-off radius, the norm of each pseudo-wavefunction is identical to its corresponding all-electron ...
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10 votes
1 answer
149 views

How does static correlation differ from Fermi correlation, and how does dynamic correlation differ from Coulomb correlation?

From what I read, the correlation energy can be grouped as static correlation and dynamic correlation. Another way to group them is Fermi correlation and Coulomb correlation. Is there any link between ...
14 votes
1 answer
696 views

When do we abandon ab initio methods?

This question is related to (and was originally asked in) another post about "quantum protectorates" I made here. Ab initio methods are nice because they directly solve a sort of "...
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12 votes
2 answers
415 views

Is there any relation between static correlation and finite-temperature smearing?

I am new to this field, so there is a chance that I am mixing two entirely different concepts together, but it seems to be that static correlation and finite-temperature smearing in DFT are somehow ...
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7 votes
1 answer
119 views

What correlation effects are included in KS-DFT with LDA and GGA?

This post is inspired by this post: What correlation effects are included in DFT? The Kohn-Sham density functional theory (KS-DFT) are considered to be exact and can include all correlation effects in ...
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15 votes
3 answers
436 views

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, ...
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11 votes
1 answer
111 views

Is it possible to make segmented contracted basis sets for correlated calculations?

The segmented contracted basis sets are usually handled well by the QM programs than generally contracted basis sets, as the primitives are not repeated. The correlation consistent basis sets (cc-PVnZ)...
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9 votes
2 answers
802 views

What are Pair Natural Orbitals?

Recently, I came across electron correlation methods that use local pair natural orbitals (LPNOs) or domain-based local pair natural orbitals (DLPNO), such as DLPNO-MP2, DLPNO-CCSD etc. The dynamic ...
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10 votes
2 answers
118 views

Quantifying electronic overlap?

I am trying to understand how to quantify the electronic interaction/conduction/overlap between two metal cores at varying distances. As someone who works primarily with classical MD, and has less ...
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7 votes
1 answer
76 views

What are the methods for calculating interaction energies of layered materials, and where are they implemented?

What are the available methods for the calculation of reference interaction energies of layered materials and in which codes they are implemented? For example, I know RPA-ACFDT in the DFT concept. But ...
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5 votes
1 answer
138 views

What are the other matrix elements in singlet symmetry-adapted CISD?

Szabo/Ostlund list the CI matrix elements between singlet symmetry-adapted configurations (SAC) in Table 4.1 of their book: $$ \langle ^1\Psi^r_ a \lvert \mathcal{H} - E_ 0 \rvert ^1\Psi^r_ a \rangle, ...
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8 votes
0 answers
60 views

For which materials is the Bethe lattice a good model to compute the density of states of the conduction band? [closed]

The Bethe lattice is exactly solvable in the limit where each lattice site has an infinite number of nearest-neighbors. In this limit it yields a semi-circular density of states in the conduction band....
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15 votes
0 answers
148 views

What are the ways to add dynamic correlation to an MCSCF wavefunction?

We recently learned about the many types of MCSCF, which treat "static" electron correlation. There are then many ways to treat further "dynamic" electron correlation to try and ...
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12 votes
1 answer
170 views

What are the practical limitations of R12/F12 methods?

Electron correlation methods based on an expansion of the $N$-electron wavefunction in terms of Slater determinants have a slow basis set convergence. Moreover, explicitly correlated methods (such as ...
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14 votes
2 answers
142 views

What are the computational limits of explicitly correlated methods?

Expanding the $N$-electron wavefunction in terms of Slater determinants (as in CI and CC theory) could lead to a very slow convergence to its basis set limit because such expansions can't give an ...
  • 2,561
27 votes
5 answers
1k views

When and Why does Density Functional Theory (DFT) fail?

Every method has its strengths and weaknesses. For instance, a strength of DFT is that is has HF like speeds, but can also account for electron-correlation and this is a pretty big feature since ...
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