# Tag Info

Accepted

### Closed-form expression for excitation energies, given the exact XC functional

Do you consider the exact frequency-dependent XC kernel as part of "the exact XC functional"? And do you allow the use of additional ingredients like two-electron integrals? If both of your ...
• 7,463

### Closed-form expression for excitation energies, given the exact XC functional

No, they cannot be obtained from a DFT calculation. As the first paper I linked in the previous response says, DFT is a ground state theory, and it should not be expected to get excited state ...
• 729
Accepted

### Question on generalised Kohn-Sham "band gap"

The optical band gap is not expected to equal the DFT band gap since excitonic effects are not included. What you are referring to is the fundamental gap. The fundamental gap is related to electron ...
• 729

### Nanowire Band Structure using Siesta DFT

This is not a complete answer with complete reasoning behind what had happened. However, I am giving this answer in case someone faces the same situation in the future. What had happened was, I did ...

### How to calculate circular dichroism in VASP?

Its a bit tough to prove a negative, but currently I believe this isn't possible with VASP or any other QM package. Searching the VASP Wiki returns no hits for "circular dichroism" or "...
• 14.3k

### Optimizing (DFT/MP2) small molecule dimers

Summarizing the OP's comment that describes how they managed to remove negative (imaginary) frequencies for their system: Ensure correct symmetry of the molecular geometry Include D3 dispersion for ...
1 vote

### Is it possible to construct a tight binding matrix of high atomic layer thin film from the lower one?

Just to preserve ProfM's comments that seem to to answer the question: In principle yes, but its accuracy will depend somehow on the specific features of your current TB model and what you want for ...
Accepted

### Different band structure in supercell, but not due to band folding

Answer: band folding is a little more subtle than I originally thought. I'd only really come across band folding in the textbook example of 1D and rather naively assumed that in higher dimensions if ...
• 131

### How to get a two-electron expectation value for atoms in PySCF?

First of all, the "diagonal" contribution of Eq. (1), $$\langle \Psi | \sum_i^N r_i^{a+b} | \Psi \rangle \tag{3}$$ is the expectation value of a one-body operator, so you already know how ...
• 7,463

### Band structure calculation

First, the lattice optimization is performed, and then the atomic position is optimized. After the stable position is obtained, the static calculation can be performed, and then the k-point path is ...

### SIESTA error: must be in PAO.Basis, (it has semicore states)

http://nninc.cnf.cornell.edu/ This URL has expired. https://departments.icmab.es/leem/SIESTA_MATERIAL/Databases/Pseudopotentials/periodictable-gga-abinit.html. Some of the pseudopotentials of this URL ...
Accepted

### Need help about how I can get/generate semicore pseudopotentials to be used in Quantum Espresso DFT?

There are many pseudopotential libraries available to choose from. Below is a (most definitely incomplete) list of pseudopotential libraries compatible with Quantum ESPRESSO that are either curated or ...
Accepted

### How does charge mixing work?

Density mixing is a type of self-consistent field (SCF) method, which tries to find the closest density to the Kohn-Sham (or Hartree-Fock etc) ground state density by mixing previous densities and ...
• 4,382