[Wien] How to use the LIBXC library?

[Gavin Abo]

> The new version of wien2k is arrived but there is no sufficient 
> information on how to use the new library LIBXC implemented

Section 7.1.3 on page 112 of the WIEN2k 16.1 usersguide [ 
http://susi.theochem.tuwien.ac.at/reg_user/textbooks/usersguide.pdf ] has:

Example for PBE with the
- global keyword : XC_PBE
- LIBXC-keywords : XC_GGA_X_PBE XC_GGA_C_PBE

The first line of case.in0 on page 110 uses the WIEN2k PBE implementation:

TOT  XC_PBE

To use the LIBXC implementation, I followed the above given example and 
changed the first line in case.in0 to:

TOT  XC_GGA_X_PBE XC_GGA_C_PBE

On page 112, it has:

Keywords to use functionals from the library of exchange and correlation 
functionals LIBXC 
(http://www.tddft.org/programs/octopus/wiki/index.php/Libxc).

Say one is interested in a GGA PW91 calculation.  So went to that page 
of the LIBXC website, clicked on the "Version 3.0" link, which directs to:

http://octopus-code.org/wiki/Libxc_3.0_functionals

There in the list for GGA PW91 is:

GGA_X_PW91
GGA_C_PW91

Also, in the list, GGA PBE is:

GGA_X_PBE
GGA_C_PBE

 From the example, it can be seen that XC_ was added to the beginning of 
it.  So first line in case.in0 for a LIBXC GGA PW91 calculation becomes:

TOT  XC_GGA_X_PW91 XC_GGA_C_PW91

> and how to combine the different functionals for exchange and 
> correlation to get a result close to the experimental one.

In the list at http://octopus-code.org/wiki/Libxc_3.0_functionals, it 
looks like there are like 50+ articles.  Each functional is usually 
designed to solve a "specific" problem or application.  So you have to 
read articles and choose an appropriate one for your application.  So 
each of those articles are basically giving their own answer to your 
question.  Also, that may also be the reason for the LIBXC library, 
nobody probably has the answer to the question for the new combinations, 
which gives researchers something new to study.