3.5 Direct interface with CASINO

PWscf supports the Quantum Monte Carlo program CASINO directly. For more information on the CASINO code, see https://vallico.net/casinoqmc. CASINO may take the output of PWSCF and 'improve it' giving considerably more accurate total energies and other quantities than DFT is capable of.

PWscf users wishing to learn how to use CASINO may like to attend one of the annual CASINO summer schools in Mike Towler's "Apuan Alps Centre for Physics" in Tuscany, Italy. More information can be found at http://www.vallico.net/tti/tti.html

3.5.0.1 Practicalities

The interface between PWscf and CASINO is provided through a file with a standard format containing geometry, basis set, and orbital coefficients, which PWscf will produce on demand. For SCF calculations, the name of this file may be pwfn.data, bwfn.data or bwfn.data.b1 depending on user requests (see below). If the files are produced from an MD run, the files have a suffix .0001, .0002, .0003 etc. corresponding to the sequence of timesteps.

CASINO support is implemented by three routines in the PW directory of the espresso distribution:

• pw2casino.f90 : the main routine
• pw2casino_write.f90 : writes the CASINO xwfn.data file in various formats
• pw2blip.f90 : does the plane-wave to blip conversion, if requested

Relevant behavior of PWscf may be modified through an optional auxiliary input file, named pw2casino.dat (see below).

3.5.0.2 How to generate xwfn.data files with PWscf

Use the '-pw2casino' option when invoking pw.x, e.g.:

pw.x -pw2casino < input_file > output_file

The xfwn.data file will then be generated automatically.

PWscf is capable of doing the plane wave to blip conversion directly (the 'blip' utility provided in the CASINO distribution is not required) and so by default, PWscf produces the 'binary blip wave function' file bwfn.data.b1

Various options may be modified by providing a file pw2casino.dat in outdir with the following format:

&inputpp
blip_convert=.true.
blip_binary=.true.
blip_single_prec=.false.
blip_multiplicity=1.d0
n_points_for_test=0
/

Some or all of the 5 keywords may be provided, in any order. The default values are as given above (and these are used if the pw2casino.dat file is not present.

The meanings of the keywords are as follows:

blip_convert

: reexpand the converged plane-wave orbitals in localized blip functions prior to writing the CASINO wave function file. This is almost always done, since wave functions expanded in blips are considerably more efficient in quantum Monte Carlo calculations. If blip_convert=.false. a pwfn.data file is produced (orbitals expanded in plane waves); if blip_convert=.true., either a bwfn.data file or a bwfn.data.b1 file is produced, depending on the value of blip_binary (see below).

blip_binary

: if true, and if blip_convert is also true, write the blip wave function as an unformatted binary bwfn.data.b1 file. This is much smaller than the formatted bwfn.data file, but is not generally portable across all machines.

blip_single_prec

: if .false. the orbital coefficients in bwfn.data(.b1) are written out in double precision; if the user runs into hardware limits blip_single_prec can be set to .true. in which case the coefficients are written in single precision, reducing the memory and disk requirements at the cost of a small amount of accuracy..

blip_multiplicity

: the quality of the blip expansion (i.e., the fineness of the blip grid) can be improved by increasing the grid multiplicity parameter given by this keyword. Increasing the grid multiplicity results in a greater number of blip coefficients and therefore larger memory requirements and file size, but the CPU time should be unchanged. For very accurate work, one may want to experiment with grid multiplicity larger that 1.0. Note, however, that it might be more efficient to keep the grid multiplicity to 1.0 and increase the plane wave cutoff instead.

n_points_for_test

: if this is set to a positive integer greater than zero, PWscf will sample the wave function, the Laplacian and the gradient at a large number of random points in the simulation cell and compute the overlap of the blip orbitals with the original plane-wave orbitals:

α = $${<BW\vert PW> \over \sqrt{<BW\vert BW><PW\vert PW>}}$$

The closer α is to 1, the better the blip representation. By increasing blip_multiplicity, or by increasing the plane-wave cutoff, one ought to be able to make α as close to 1 as desired. The number of random points used is given by n_points_for_test.

Finally, note that DFT trial wave functions produced by PWSCF must be generated using the same pseudopotential as in the subsequent QMC calculation. This requires the use of tools to switch between the different file formats used by the two codes.

CASINO uses the `CASINO tabulated format', PWSCF uses the UPF format. See upflib/README.md for instructions on how to convert between these formats.

An alternative converter `casinogon' is included in the CASINO distribution which produces the deprecated GON format but which can be useful when using non-standard grids.