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19 votes
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Why is my hydrogen energy not equal to -0.5 hartrees?

Yes, the exact orbital energy of the hydrogen atom is $-1/2\ E_h$. The problems in the calculation are as follows. First, you are running ROKS. ROKS has many definitions in the literature; PySCF ...
Susi Lehtola's user avatar
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14 votes
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pyscf: Why should the choice of pseudopotential determine the total energy, and not the physics?

The energy of a system, be it a molecule or a material, is dependent on the energy reference (or energy zero point) you choose. Common energy references are: The state where all electrons (except for ...
wzkchem5's user avatar
  • 9,670
12 votes

Why is my hydrogen energy not equal to -0.5 hartrees?

The exact non-relativistic ground state electronic energy of a hydrogen-like atom, assuming that the nucleus is "clamped", point-sized, and with an infinitely large mass compared to the mass ...
Nike Dattani - No Free Time's user avatar
10 votes
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Are there non-antisymmetric solutions to the electronic Hamiltonian?

So, your question is "are the solutions of the Schrödinger equation guaranteed to be anti-symmetric functions, or is this an ad hoc assumption resulting from knowledge of the anti-symmetry ...
Susi Lehtola's user avatar
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8 votes
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How may I solve the radial Schrödinger equation?

You have not specified sufficiently precisely what is the problem you want to solve. It sounds like you want to solve the radial density functional equations; however, there is still a world of ...
Susi Lehtola's user avatar
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8 votes

How may I solve the radial Schrödinger equation?

Solving the radial Schroedinger equation (your Eq. 1) is done with $V(r)$ pre-determined (either it is an analytic expression of a model, such as the Morse/long-range model, or it is known in some ...
Nike Dattani - No Free Time's user avatar
8 votes
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What are alpha and beta electrons?

Alpha and beta basically denote spin-up (+1/2 for electrons) and spin-down (-1/2 for electrons). This tells us how the spin would align if we have a magnetic field. A spin of +1/2 actually means that ...
Nike Dattani - No Free Time's user avatar
8 votes
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Which is better for binding energies: DFT-D3 or RI-MP2?

Summary of my recommendation If my choices are B3LYP-D3 and RI-MP2 for calculating a binding energy at a molecule whose geometry was optimized with B3LYP-D3, I would find it easier to justify using ...
Nike Dattani - No Free Time's user avatar
8 votes
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Python library to obtain atomic orbitals of atoms in the lowest energy state

Mendeleev You can use a python library called Mendeleev. Below is an example of a Silicon atom. ...
Vandan Revanur's user avatar
8 votes
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Materials with low band gap and low conductivity simultaneously. Is it possible?

There are such materials! See chp. 11 in Girvin-Yang which gives a much better discussion than I am providing here. You could probably also look up 'Anderson insulator' online or in any other solid-...
Tyler Sterling's user avatar
7 votes

Quantifying relativistic effects in DFT calculations

In reality, the contribution to an energy due to relativity, would be the difference between the "exact" energy with a model that captures all relativistic effects, versus a model that ...
Nike Dattani - No Free Time's user avatar
6 votes

How do I obtain CB and VB values from Quantum ESPRESSO?

I would recommend to obtain VB top and CB bottom energy values from the output file. The occupation numbers are listed if 'verbose' mode is active. In any output mode the energies should be printed ...
Leon A's user avatar
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6 votes

Why is electron correlation called the "chemical glue"?

Coincidentially, I recently read Perdew's recent autobiographical article where he also mentions the concept of "chemical glue." His simple image is as follows: The exchange–correlation ...
Kristof Bal's user avatar
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6 votes
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Why use PySCF/OpenMolcas instead of VASP/QuantumESPRESSO

The main difference is that PySCF, OpenMolcas, and other popular software such as Gaussian, ORCA, Molpro, Psi4, MRCC, CFOUR, Dalton, GAMESS, COLUMBUS, ACES, etc., are very well-developed for using ...
Nike Dattani - No Free Time's user avatar
6 votes
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what determines the 'Estimated total dynamical RAM ' in a Quantum Espresso calculation

When people talk about the scaling of a conventional DFT implementation being cubic with simulation size, they are talking about the computational time, not the RAM required. The RAM actually scales ...
Phil Hasnip's user avatar
  • 8,132
6 votes

Are there non-antisymmetric solutions to the electronic Hamiltonian?

There are several things to notice here. First, the Hamiltonian you wrote is not a completed definition of an operator, since it lacks boundary conditions. This means that there's no eigenproblem yet. ...
Ruslan's user avatar
  • 161
5 votes
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How do I get the energy, if I have the wavefunction?

"For a known wave function, should I not be able to determine the energy?" The wavefuncton is an eigenfunction of the Hamiltonian, and the energy is a corresponding eigenvalue of the same ...
Nike Dattani - No Free Time's user avatar
5 votes

Can the the relaxed energy of a material system (eV/atom) computed by DFT using VASP be positive?

Firstly, by expressing the energy per atom (eV/atom), it allows for a more convenient comparison between different materials or systems, as it provides a measure of the energy on a per-atom basis. ...
Jaafar Mehrez's user avatar
5 votes

How do I obtain CB and VB values from Quantum ESPRESSO?

From the graph you presented: Select the Brillouin $\Gamma$ point, $G$ in the graph (this is for a direct gap system). Look for where the Fermi level is. In the graph, it is indicated by the red ...
Camps's user avatar
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4 votes

Has non-orthogonal CI (NOCI) been implemented in PySCF?

NOCI is not available in the main branch of PySCF That is why your question on the PySCF GitHub page which you asked on the same day as this question (next time please let us know if you are "...
Nike Dattani - No Free Time's user avatar
4 votes
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What do we get by integrating over the charge density data of a material system calculated with DFT?

The integration of the charge density over the entire system gives you the total charge of the system. This value should be equal to the total number of electrons in the system, accounting for both ...
Jaafar Mehrez's user avatar
4 votes
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'The combination of multiplicity electrons is impossible' in Gaussian

"The combination of multiplicity 1 and 219 electrons is impossible." 219 is an odd number of electrons. A spin multiplicity of 1 means that there are no unpaired electrons. This means that ...
Nike Dattani - No Free Time's user avatar
4 votes
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Can the the relaxed energy of a material system (eV/atom) computed by DFT using VASP be positive?

When DFT is being used for electronic structure energy calculations, a negative energy would be indicative of bound electrons. There's nothing physically, chemically or mathematically illegal about ...
Nike Dattani - No Free Time's user avatar
4 votes
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Plane-wave basis expansion for periodic material modelling: cutoff energy inequality and number of atoms in the simulation

You’re quite right: as written, the equation is dimensionally inconsistent. There are actually a bunch of constants that make the units work out, but it is customary in matter modeling to define units ...
elutionary's user avatar
  • 1,006
4 votes

Different results from MP2 calculations on same geometry in ORCA

Have you confirmed that the geometries for each are exactly the same? If the second calculation took a single step, its possible it moved the geometry very slightly. ORCA's default convergence ...
Tyberius's user avatar
  • 15.6k
4 votes

Are there non-antisymmetric solutions to the electronic Hamiltonian?

It depends on what you mean. If your Hamiltonian is defined on $H_N:=\otimes^N \mathfrak h$, the $N$-fold tensor product of the single-particle Hilbert space $\mathfrak h$, then actually under quite ...
Jakob's user avatar
  • 757
3 votes

How do I get the energy, if I have the wavefunction?

UPDATE: I didnt read your post carefully enough. My answer is irrelevant. In your comment above you said I guess what i meant was that if I have an arbitrary state eigenfunction (approximated as the ...
Tyler Sterling's user avatar
3 votes

Why is electron correlation called the "chemical glue"?

Normally the correlation energy is defined to be: $E_{corr}^{basis} = E_{FCI}^{basis} - E_{HF}^{basis}$ where the superscript basis denotes each calculated in a fixed basis set. The correlation energy ...
lex2763's user avatar
  • 391
3 votes

How to obtain imaginary frequency polarizability?

I dedicate this answer to Bob Le Roy and Jim Mitroy who passed away in August 2018 and August 2014 respectively. I was thinking about them a lot while writing this answer. Dynamic dipole ...
Nike Dattani - No Free Time's user avatar
3 votes
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Computing proton-proton potentials for infinite periodic materials

In electronic structure calculations for infinite periodic materials, the proton-proton energy is typically not directly computed or used to monitor the convergence of the self-consistent field (SCF) ...
Jaafar Mehrez's user avatar

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