Difference between "bands" and "nscf" calculations in quantum espresso?
Dear:
in the case of nscf calculations, we calculate the eignvalues of a given system for a well defined k-point grid starting from previuos scf calculations. in the case of 'bands' calculations, we calculate the eignvalues of this system along a well defined k path in the IBZ region.
best regards
It does not matter what method (bands or nscf) we use to calculate the band structure?
they work as the same way for calculation of band structure but If you are interested in calculating only the Kohn-Sham states for the given set of k-points, use calculation=’bands’. If you are interested in further processing of the results of non-SCF calculations(for instance, in DOS calculations) use calculations=’nscf’.
in the scf calculation, at each scf step, you perform iterative diagonalizations starting from the wavefunctions of the previous scf step, using a convergence criterion for diagonalization that is very loose at the beginning, gets tighter as you approach self-consistency. In this way, you perform several scf steps, each one requiring diagonalizations that converge quickly. In the non-scf calculation, you make a single step but you have to start from superposition of atomic orbitals and use a tight convergence criterion for diagonalization. Another factor is the number of bands used: in scf calculations one typically uses only occupied bands; in non-scf calculation, one typically is interested in empty bands as well. This makes each diagonalization much costlier in the non-scf case than in the scf case. So typically a non-scf calculation may be faster (per k-point) than a scf one, but not by much; sometimes not at all, or even slower. Have a look at the number of H*psi made in the two calculations and at the time spent there. H+psi is a basic ingredient of iterative diagonalization, and typically takes the bigger slice of computer time. The implementation of non-scf calculations is currently non-optimal, because we throw away sll the information from scf except the potential, but since non-scf is seldom the dominant part of a realistic calculation, little effort has been devoted to its further optimization.
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