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21 votes

How does the Hartree-Fock method improve on Hartree after considering the wavefunction that takes the anti-symmetry property into account?

Both Hartree and Hartree-Fock have a self-consistent field method, as does density functional theory, too; the difference is just that Hartree theory uses a bosonic wave function (symmetric with ...
Susi Lehtola's user avatar
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20 votes

What is nscf calculation in Quantum ESPRESSO?

NSCF stands for non-self-consistent field calculation and, as explicit by its name, the calculation is not performed in a self-consistent fashion as the SCF (self-consistent field) one. The latter ...
Anibal Bezerra's user avatar
16 votes

How does the Hartree-Fock method improve on Hartree after considering the wavefunction that takes the anti-symmetry property into account?

The procedure to find the Hartree equations and the Hartree-Fock equations is very similar, we have to minimize the expectation value of the Hamiltonian under the orthonormalization constraint. ...
Verktaj's user avatar
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16 votes
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What is nscf calculation in Quantum ESPRESSO?

The central goal of KS-DFT is solving Kohn-Sham equation: $$H\psi_i(\vec{r})=\left( -\dfrac{\nabla^2}{2}+V_{ks}[\vec{r};\psi_i(\vec{r})] \right)\psi_i(\vec{r})=E_i\psi_i(\vec{r})$$ Here the atomic ...
Jack's user avatar
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15 votes
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What are the physical reasons if the SCF doesn't converge?

I have written a section in the BDF user manual on this issue. It is in Chinese but I'll roughly translate the key parts to English as below. Common reasons for the SCF procedure to fail to converge ...
wzkchem5's user avatar
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13 votes
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What programs can do electron integrals and SCF calculations with more than one CPU?

Psi4 and PySCF Psi4 and PySCF are free, open source programs that can do both parallel integrals and SCF. MPI parallellization is quite rare in quantum chemistry codes, since the algorithms don't tend ...
Susi Lehtola's user avatar
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13 votes

Difficult cases for converging Kohn-Sham SCF calculations

A few materials/simulation boxes I've had some proper trouble with: HSE06 + noncollinear magnetism + antiferromagnetism, Vasp noncollinear: This was a strongly antiferro material (4 Fe atoms, in an ...
Anubhab Haldar's user avatar
13 votes
Accepted

One-center two-electron integrals between 1s STO

Your Octave code is trying to do the integral by quadrature, which makes very little sense since it will have a huge problems with the cusp. Since this is a one-center problem, the best approach is ...
Susi Lehtola's user avatar
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12 votes

What is the mathematical condition that ensure that the self-consistent field (SCF) procedure must converge?

This question is a bit ill-defined: what do you mean by "the self-consistent field procedure"? If you mean the original Roothaan procedure, then the question makes sense, but it is uninteresting: ...
Susi Lehtola's user avatar
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12 votes

What are the types of SCF?

GHF: Generalized Hartree Fock In Restricted Hartree-Fock (RHF), the molecular orbitals are constructed as pairs, with a single spacial function being used to describe both an $\alpha$ and $\beta$ spin ...
Tyberius's user avatar
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12 votes
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How to make sure that my SCF calculation is correct?

First, make sure that the structure is correct. Second, make sure that the structure is relaxed. Third, make sure that all the parameters in your input card are reasonable and the used ...
Jack's user avatar
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11 votes

Standard values for level of convergence

This is an excellent question! The reality is complicated even in LCAO calculations: every code has different defaults, which also depend on the run type. It seems that older LCAO codes simultaneously ...
Susi Lehtola's user avatar
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11 votes
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What are the types of SCF?

DHF: Dirac-Hartree-Fock (or "Dirack-Fock") the DHF (Dirac-Hartree-Fock) or Dirac-Fock is the SCF method based upon four-component spinors (simply four-spinors), because of the four-component ...
Miro Iliaš's user avatar
11 votes
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Electronic structure calculations without SCF?

Yes! The Schroedinger equation for a molecule is just a differential equation, so you can solve it numerically "on a grid" or in "real-space", and there is a review paper on the ...
Nike Dattani - No Free Time's user avatar
11 votes
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For DFT and Hartree-Fock, how can we know that we have a true minimum? Is there an equivalent to the "frequency analysis" for geometry optimization?

You're right about the ability to change the initial guess repeatedly until you get the lowest energy, and this is how it's done in software like MOLPRO which don't offer "stability analysis"...
Nike Dattani - No Free Time's user avatar
11 votes

For DFT and Hartree-Fock, how can we know that we have a true minimum? Is there an equivalent to the "frequency analysis" for geometry optimization?

Just like geometry optimization, there is no practical way to be 100% sure that you have the global minimum of SCF solutions. But there are checks you can do to make sure that the SCF solution you got ...
S R Maiti's user avatar
  • 7,001
10 votes

Difficult cases for converging Kohn-Sham SCF calculations

This will be a long answer, so I will divide it in parts. Woods paper A significant limitation of the Woods et al paper is that it excludes atomic-basis set calculations where convergence ...
Susi Lehtola's user avatar
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10 votes
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What are the types of MCSCF?

LASSCF: Localized Active Space SCF An approximation to or generalization of (depending on how you look at it) CASSCF. In CASSCF, the wave function consists of an antisymmetrized product of two factors ...
Matthew R Hermes's user avatar
9 votes

What are the types of SCF?

RHF: Restricted Hartree-Fock / RKS: Restricted Kohn-Sham Restricted Hartree-Fock (RHF) is a self-consistent field approach: a mean-field approximation to the electronic, non-relativistic Schrödinger ...
TAR86's user avatar
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9 votes
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What do the rows and columns of a Fock matrix represent?

The short answer is: it is the matrix representation of the Fock operator in the given basis set, in this case, the atomic orbital (AO) basis. The Fock operator itself is a mean-field, independent ...
TAR86's user avatar
  • 1,697
9 votes

One-center two-electron integrals between 1s STO

This can be solved analytically, a complete solution can be found here To refrain from rewriting the entire derivation I will only say that you need to integrate over all 3 dimensional degrees of ...
Cody Aldaz's user avatar
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9 votes
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Getting different total magnetic moment in 'scf' and 'vc-relax' calculations in Quantum ESPRESSO

Looking at your input and output files, I think the most likely explanation is a different local minimum in the energy, as mentioned by Andrew in the comments. If you look at the final scf step in the ...
Kevin J. May's user avatar
  • 3,161
9 votes

Getting different total magnetic moment in 'scf' and 'vc-relax' calculations in Quantum ESPRESSO

Edit: This no longer seems to be the issue, as the atomic positions are apparently essentially unchanged, per @KevinJ.M.'s comment. I will nonetheless keep the answer here because it is still ...
Andrew Rosen's user avatar
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9 votes

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 ...
Nike Dattani - No Free Time's user avatar
9 votes

Which is the recommended way to optimize "big" systems?

Suggestions for Molecular-Like Systems (not periodic unit cells) I'll make some general suggestions for molecular / isolated systems, not crystals. (In the case of periodic systems, some methods aren'...
Geoff Hutchison's user avatar
9 votes
Accepted

SCF energy keeps on fluctuating between two values, instead of converging, why?

The cause of the oscillations in the SCF energies is probably a "sloshing instability"; the two most common types are "charge sloshing" and "occupancy sloshing". Sloshing ...
Phil Hasnip's user avatar
  • 7,907
8 votes

Which SCF energy to use to plot PES after calculations with SMD?

Generally you want to use the same settings when trying to combine results from different jobs. However, below is the general procedure that I have seen performed for a basic, publishable PES. 1) ...
kskinnerx16's user avatar
  • 1,546
8 votes

What is the mathematical condition that ensure that the self-consistent field (SCF) procedure must converge?

If by "the SCF method" you mean the simple SCF, the answer is: no, usually it does not converge (unless the problem is very simple; basically the gap is huge, so that the system does not respond very ...
Antoine Levitt's user avatar
8 votes
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Normalization constant and Roothaan Equations

The normalization constant is built into the Roothaan-Hall equation. Namely, when you derive the Hartree-Fock energy expression from the wave function, you integrate out all the orbitals that don't &...
Susi Lehtola's user avatar
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8 votes
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How is "basis set projection" done?

Supplementing Nike's answer above.. This is actually quite elementary math. Let's say I have an orbital in a basis $|\psi\rangle = \sum_i c_i |i\rangle$ and I want to find the expansion in some other ...
Susi Lehtola's user avatar
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