[CP2K-user] DF2 van der Waals (vdW) and XC functionals

[Ronald Cohen]

I am quite confused about the (lack of) documentation for the XC 
functionals and non-local vdW. I find examples such as:

&XC

&XC_FUNCTIONAL NO_SHORTCUT

&LIBXC T

FUNCTIONAL XC_GGA_X_RPW86

&END LIBXC 

&VWN T 

&END VWN

&END XC_FUNCTIONAL &VDW_POTENTIAL

POTENTIAL_TYPE NON_LOCAL &NON_LOCAL

TYPE LMKLL

VERBOSE_OUTPUT T KERNEL_FILE_NAME ./vdW_kernel_table.dat CUTOFF 160

&END NON_LOCAL &END VDW_POTENTIAL

&END XC


(from 
http://archer.ac.uk/training/course-material/2014/08/CP2K/Slides/NSCCS_2014_Sanliang_Ling.pdf
 )


and similar even in the regtests.

This seems to call both XC_GGA_X_RPW86 and VWN (lda) .

What does it mean to call two different xc potentials in cp2k, and why 
would one do it?

Tracing this in the code is not so easy either.


Ins eom docs it seems to suggest that LDA only should be used for 
correlation. Is that because GGA is included 

in the vDW function in cp2k? Also there is the question which 
pseudopotentials to use.


It seems to me that  XC_GGA_X_RPW86 is correct without the VMN, and this 
would correapond to what is done for DF2 in

quantum espresso it seems.


I note some related problems with the documentation. I find examples like 
for LIBXC:

FUNCTIONAL XC_MGGA_X_M06_L XC_MGGA_C_M06_L

but the docs at 

say

   - FUNCTIONAL 
   <https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/DFT/XC/XC_FUNCTIONAL/LIBXC.html#list_FUNCTIONAL>
   - This keyword cannot be repeated and it expects precisely one word.
   
Yet there are "two words" 

and including VWN afterwards also seems like more than one functional, as I 
said above.


Thank you!


Ronald Cohen


 
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