K-Points and Extrapolation

[M. Brehm]

Dear community,

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 
be fixed.

(*) 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,
Martin

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