21
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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 ...
- 20k
21
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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 ...
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17
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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 ...
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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. ...
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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 ...
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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 ...
- 20k
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 ...
- 1,436
13
votes
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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 ...
- 20k
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: ...
- 20k
12
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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 ...
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12
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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 ...
- 15.3k
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 ...
- 20k
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 ...
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11
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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 ...
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"...
11
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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?
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 ...
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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 ...
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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 ...
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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 ...
- 1,698
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 ...
- 1,698
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 ...
- 8,015
9
votes
Accepted
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 ...
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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 ...
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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 ...
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'...
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9
votes
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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 ...
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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) ...
- 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 ...
- 646
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 &...
- 20k
8
votes
What do the rows and columns of a Fock matrix represent?
Formally, the Fock matrix is the density matrix derivative of the Hartree-Fock or Kohn-Sham energy functional. The Fock matrix returned by PySCF is in the atomic-orbital basis, which is actually the ...
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