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2.6 Libxc library

QUANTUM ESPRESSO is compatible with libxc version 4.3.0 or later (compatibility with older versions is not guaranteed).
The libxc functionals are available for LDA, GGA and metaGGA, however, not all of them are straightforwardly usable. Some of them may depend on specific external parameters and some others may provide as output the energy or the potential, but not both. Therefore some attention has to be paid when using libxc. Warning messages should appear in the output when particular cases whose correct operation in QUANTUM ESPRESSO is not guaranteed are chosen.

2.6.1 Linking in QUANTUM ESPRESSO

Once installed libxc, the linking with QUANTUM ESPRESSO can be enabled directly through the configuration script by adding the two switches –with-libxc and –with-libxc-prefix, e.g.:
./configure --with-libxc --with-libxc-prefix='/path/to/libxc/'
By adding the first switch only an automatic search for the libxc folder will be attempted, but its success is not guaranteed. It is always preferable to specify the second switch too. Optionally, a third switch can be added, namely –with-libxc-include='/path/to/libxc/include', which specifies the path to the Fortran headers (usually it is not necessary).

Alternatively, the link to libxc can be activated after the configuration of QUANTUM ESPRESSO by modifying the file in the main folder in this way:

Then QUANTUM ESPRESSO can be compiled as usual. Note for version 5.0.0:

if the version of libxc is 5.0.0, the last point must be replaced by: since the f03 interfaces are no longer available. They have been restored in following releases. Version 5.0.0 is still usable, but, before compiling QUANTUM ESPRESSO, a string replacement is necessary, namely `xc_f03' must be replaced with `xc_f90' everywhere in the XClib folder.

2.6.2 Usage

In order to use libxc functionals, you can enforce them from input by including the input_dft string in the system namelist. Starting from v7.0 of QUANTUM ESPRESSO the only allowed notation for DFTs that include Libxc terms is the index one. For example, to use the libxc version of the PBE functional (both exchange and correlation):
input_dft = `XC-000I-000I-101L-130L-000I-000I'
The letters I or L next to each ID stand for Internal and libxc. This is equivalent to the old full-name notation:
input_dft = `gga_x_pbe gga_c_pbe'   ***OLD***
The order must be the usual one, namely LDA exchange, LDA correlation, GGA exchange, GGA correlation, MGGA exchange, MGGA correlation. libxc exchange+correlation functionals can be put in the exchange or in the correlation slot with no difference.
The reason why the full-name notation has been disabled is to eliminate the risk of overlaps among different names (occurring especially when combinations of internal and libxc DFTs are used).
The complete list of libxc functionals (and their IDs) is available at:
Combinations of QUANTUM ESPRESSO and libxc functionals are allowed in PW, but some attention has to be paid to their reciprocal compatibility (see section below).
For example, the internal exchange term of PBE together with the correlation term of PBE in libxc is obtained by:
input_dft = `XC-001I-000I-003I-130L-000I-000I'
which corresponds to the old:
input_dft = `sla pbx gga_c_pbe'   ***OLD***
Note that when using GGA internal functionals you must always specify the LDA term too, while it is not the case for the libxc ones.
Abbreviations are allowed when zero tails are present. The above example is still valid by putting:
input_dft = `XC-001I-000I-003L-130L'
since no MGGA terms are present.
Non-local terms can be included by just adding their name after the index notation, for example:

2.6.3 Differences between Libxc and internal functionals

There are some differences between QUANTUM ESPRESSO functionals and libxc ones. In QUANTUM ESPRESSO the LDA and the GGA terms are separated and must be specified independently. In libxc the GGA functionals already include the LDA part (Slater exchange and Perdew&Wang correlation in most of the cases with the exception, for example, of Lee Yang Parr functionals).
The libxc metaGGA functionals may or may not need the LDA and GGA terms, depending on the cases, therefore a good check of the chosen functionals is recommended before doing expensive runs.
Some functionals in libxc incorporate the exchange part and the correlation one into one term only (e.g. the ones that include the `_xc' kind label in their name). In these cases the whole functional is formally treated as `correlation only' by QUANTUM ESPRESSO. This does not imply any loss of information.

2.6.4 Special cases

A number of libxc functional routines need extra information on input and/or provide only partial information on output (e.g. the energy or the potential only). In these cases the use of such functionals may not be straightforward and, depending on the cases, may require some work on the QUANTUM ESPRESSO source code. External parameters.

Several functionals in libxc depend on one or more external parameters. Some of these can be recovered inside QUANTUM ESPRESSO, some others are not directly available. In all these cases a direct intervention on the QUANTUM ESPRESSO source code might be necessary in order to be able to properly use such functionals. However two routines have been defined in the XC library of QUANTUM ESPRESSO that ease the task of setting and recovering the external parameters in libxc: In order to see the available parameters for a given libxc functional and their corresponding indexes, the xc_infos.x program is available in XClib folder. For more details see Sec. 2.6.5.
The two routines can be called almost anywhere in QUANTUM ESPRESSO, however, as any other XClib setting routine, they must be declared through the xc_lib module.
Without setting the external parameters inside the code, their default value will be assumed. This could lead to results different from the expectations.
In any case, when external parameters are needed by the chosen functionals, a warning message will appear in the output of QUANTUM ESPRESSO. An example of Libxc parameter setting can be found in the xclib_test.f90 code (see below). Functionals with partial output.

A few libxc functional routines provides the potential but not the energy. These functionals are available in QUANTUM ESPRESSO for all the families and their output energy is set to zero. MGGA Functionals that depend on the Laplacian of the density.

At present such functionals are formally usable in QUANTUM ESPRESSO , but their dependency on the Laplacian is ignored and the corresponding output term of the potential is set to zero. Since the Laplacian of the density is computable in QUANTUM ESPRESSO, they might be fully exploited with a limited intervention on the code. Other functionals.

Besides exchange (_x), correlation (_c) and exchange plus correlation (_xc), a fourth kind of functionals is available in libxc, the kinetic functionals (_k). At present, they are not usable in QUANTUM ESPRESSO.

2.6.5 XC test

A testing program, xclib_test.x, for the XClib library of QUANTUM ESPRESSO is available. The program is available for LDA, GGA and MGGA functionals (both QE and Libxc). It also tests the potential derivatives for LDA (dmxc) and GGA (dgcxc).
Another small program, xc_infos.x, is available in the XClib folder starting from v6.8. It receives as input the name of any DFT usable in QUANTUM ESPRESSO (both internal and libxc) and provides infos about their family, type, external parameters, limitations, references, etc.
See XClib/README.TEST file for further details on each of the two programs.

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