Dear Paolo, Thank you very much for detailed explanation.
Trinh -----Original Message----- From: pw_forum-bounces at pwscf.org [mailto:[email protected]] On Behalf Of Paolo Giannozzi Sent: Friday, September 03, 2010 6:42 AM To: PWSCF Forum Subject: Re: [Pw_forum] Nscf and scf Vo, Trinh (388C) wrote: > I am wondering how can it take more time for nscf calculation > than for scf calculation. 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. P. -- Paolo Giannozzi, Democritos and University of Udine, Italy _______________________________________________ Pw_forum mailing list Pw_forum at pwscf.org http://www.democritos.it/mailman/listinfo/pw_forum
