CASINO manual - Theory of Condensed Matter

8 Generating CASINO trial wave functions with other programs
The following third-party codes have support for producing casino trial wave function xwfn.data
files. The exact degree of support will depend on time; in particular the interfaces may stop working
if the developers release a new version of their code without bothering to check that casino support
still works. If you find this is the case, please let us know. All voluntary contributions to maintaining
these interfaces is gratefully accepted, since doing so is very boring indeed. Our personal support
for the codes can be greatly enhanced if we have someone working in Cambridge who is a dedicated
user of the code in question. When they leave, our group knowledge may become non-existent (e.g.,
currently, none of us know anything about the Gaussian code).
Example input files for some of the codes below can be found in the distribution directory examples/wfn
generation. Instructions are in accompanying READMEs.
A summary of the current status of the various interfaces—which supplements and may be
more up-to-date than the information given here—can be found on the casino web site here:
http://vallico.net/casinoqmc/interfaces/
and a page of advice for developers who wish to
http://vallico.net/casinoqmc/interface_development/
create their own interfaces is here:
Developers should note that for Gaussian basis set codes, there is a special test suite in the directory
examples/generic/gauss dfg. The README file there should be consulted for conventions on solid
harmonics/normalization etc., and calculations should be run using the provided set of input files to
test the treatment of d, f and g functions.
8.1 ABINIT
Website: www.abinit.org.
To produce a wave function suitable for use as a casino trial wave function, certain abinit parameters
must be set correctly. To tell abinit to write the wave function in casino format, set prtwf to 2
in the abinit input file. casino (and QMC methods generally) can only take advantage of timereversal
symmetry, and not the full set of symmetries of the crystal structure. Therefore, abinit
must be instructed to generate k points not just in the irreducible Brillouin zone, but in a full half
of the Brillouin zone (using time-reversal symmetry to generate the other half). Additionally, unless
instructed otherwise, abinit avoids the need for internal storage of many of the coefficients of its
wave functions for k points that have the property 2k = G latt , where G latt is a reciprocal lattice
vector, by making use of the property that c k (G) = c ∗ k (−G − G latt). abinit must be instructed not
to do this in order to output the full set of coefficients for use in casino. See the abinit theoretical
background documents ABINIT/Infos/Theory/geometry.pdf and ABINIT/Infos/Theory/1WF.pdf
for more information.
The first of these requirements is met by setting the abinit input variable kptopt to 2 (see
ABINIT/Infos/varbas.html#kptopt) and the second by setting istwfk to 1 for all the k points (see
ABINIT/Infos/vardev.html#istwfk). Since casino is typically run with relatively small numbers
of k points, this is easily done by defining an array of ‘1’s in the input file.
For example, for the eight k points generated with ‘ngkpt 2 2 2’, we add the following lines to the
abinit input file:
# Turn off special storage mode for time-reversal k-points
istwfk 1 1 1 1 1 1 1 1
# Use only time reversal symmetry, not full set of symmetries.
kptopt 2
# Write a CASINO pwfn.data wave function file
prtwf 2
Other useful input variables of relevance to the plane waves abinit will produce include ecut,
nshiftk, shiftk, nband, occopt, occ, spinat and nsppol (see relevant input variable documents
in ABINIT/Infos/). If abinit is run in multiple dataset mode, the different wave functions for the
various datasets are exported as pwfn1.data, pwfn2.data, . . . , pwfnn.data where the numbers are
the contents of the contents of the input array jdtset (defaults to 1,2,. . . ,ndtset).
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