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I have been using 'nscf' for DOS calculation since it is faster and therefore possible to use a higher kpoints. But why is it faster? why not use it in the place of 'scf'?

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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 unit has been adopted. Note that the Kohn-Sham equation is a nonlinear differential equation and hence we need to solve it self-consistently. The workflow can be summarized as the following:

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Once the self-consistent calculation is done then the converged charge density is obtained.

With this converged charge density at the previous self-consistent run, you can always construct the KS Hamiltonian once again and diagonalize it to obtain eigenvalues along the assigned K path in reciprocal space or calculate the density of states on a denser uniform k mesh. Note that you just read the converged charge density to construct the Hamiltonian without any update for the charge density, that's the meaning of NSCF.

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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 performs the solution trying to minimize the density charge functional until a predetermined limit in the energy difference between two consecutive steps. The convergence against the k-points mesh should also be achieved. Therefore the SCF calculation should be performed first to ensures the minimum Khom-Sham energy state that should resemble the system's ground state.

Calculations such as the Band structure, Density of States, and optical properties, in general, require a denser grid at the reciprocal space. Thus the NSCF calculation should be performed after the SCF one, sampling the system to a denser mesh in the reciprocal space, allowing for the aforementioned calculations.

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  • $\begingroup$ In case of Quantum ESPRESSO, I believe Band structure calculation takes the structure will only take data generated from 'scf' so doing a 'nscf' calculation will no effect band structure does it? $\endgroup$ – Ashique Lal Oct 27 at 14:09
  • $\begingroup$ Imagine calculating a sine curve then needing additional accuracy. You can either interpolate between points or calculate the missing points. NSCF is basically the same as calculating the missing points. It can effect the shape of the band structure some but its unlikely to change things dramatically. $\endgroup$ – Tristan Maxson Oct 27 at 15:30

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