K-Points and Extrapolation
brehmin... at gmail.com
Wed Aug 22 13:40:58 CEST 2018
since a few years, CP2k supports K points in DFT calculations, which is a
very useful feature. All of the REGTESTs which invlove K points use
"EXTRAPOLATION USE_GUESS". For single-point calculations, this is not a
problem. However, for geometry optimizations or molecular dynamics runs,
this means that the initial guess is performed in every geometry step, and
the previous optimized wave function is not at all considered.
In very simple test cases, it is easy to reach SCF convergence from the
initial guess. However, in realistic solid state systems (such as
semiconductors), it can be very hard to converge a wave function from the
initial guess. This can involve hundreds of SCF cycles, and can even
require manual switching of the level of theory (e.g., pre-converge the
wave function with PBE, then use this as a starting point for PBE0). If the
initial guess is performed in every geometry step, such systems often
cannot be treated at all...
I had a short look in the source code, and saw that two other EXTRAPOLATION
methods are allowed in combination with K points, namely "LINEAR_P" and
"USE_PREV_RHO_R". However, with the first one, I obtain a SEGFAULT every
time in geometry optimization, and with the second one, the SCF convergence
is even worse than with using the initial guess.
So here are my two questions:
(*) Are these two extrapolation methods supposed to work with K points?
Maybe I am just doing something wrong, or there is a small bug which could
(*) If not: Would it be possible to implement a method to just use the last
converged wave function for new geometry steps? No sophisticated
extrapolation scheme is required. It would be a huge gain if it would be
possible to simply copy the last converged wave function. From my (very
limited) understanding of the K point implementation in CP2k, this should
not be too hard to implement...
Thank you in advance and with best regards,
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