Casino2Castep

This program reads an electron density (hereafter, simply density) from CASINO stored as the Fourier components at a set of G-vectors in reciprocal space and then performs an FFT to obtain the real space density on a grid that can then be read in CASTEP.

Installation

Pre-requisites

In order to compile this program, you will require:

• Fortran compiler, currently only the GNU compiler gfortran and Intel ifort have been tested/supported. If you wish to use another compiler, see below.

• FFTW3 must be installed. In particular, make sure you have installed the double precision version of FFTW3 and not the single (float) and long-double precision versions.

• GNU Make (gmake). In principle any version of Make should work but compilation has been tested on version 4.4.0.

Compilation

The easiest way to install is to use the Makefile.

1. Clone or download the repository.

2. Open the Makefile

3. Set the F90 variable to your compiler (either gfortran or ifort). Then set the BUILD variable to fast.

4. Now run the command: make

The executable will be located in the root file directory of the repository. If you wish to recompile from scratch, first run make clean then repeat the above steps.

Known Issues

Currently, compilation will fail if Make is run in parallel using the -j flag.

Usage

The program is capable of understanding CASTEP input files and parsing them. Therefore, the same input parameters you use for CASTEP should work here.

The most basic usage is castep2casino <casino_file> [lat_geom_file] where casino_file contains the CASINO density.

If you do not specify the lat_geom_file, the program will look for it in the form of in the form of seedname.latt_geom. The seedname will be taken from the casino_file, i.e. if the CASINO file is named QMC.txt, the program will look for the file QMC.latt.

Note that keywords are case-insensitive.

User Options

Summary

The <lat_geom_file> effectively contains the set of user-defined parameters which are:

• prim_latt_cart block - primitive real lattice vectors.
• castep_grid = the dimensions of the CASTEP grid.
• unit_bohr - unit for real lattice vectors are Bohr. This is an OPTIONAL parameter, and otherwise angstroms are used as the input unit.
• output_file - The file to use to write the CASTEP output. This is an OPTIONAL parameter.

Essential

The primitive real lattice vectors are specified as follows:

%block prim_latt_cart
a_x a_y a_z
b_x b_y b_z
c_x c_y c_z
%endblock prim_latt_cart

The default input unit for the lattice vectors are ANGSTROMS. You may alternatively put the string unit_bohr anywhere in the cell file and then enter the lattice vectors in atomic units (Bohr).

The second thing you must specify is the required CASTEP grid size. This is likewise done in the lat_geom_file through the parameter

castep_grid <nx> <ny> <nz>

For instance, if you needed a grid of size 10 x 20 x 30, you would enter castep_grid 10 20 30.

For the output file, if output_file is not specified, the program will use the lat_geom_file seedname. For example, if you have a file named QMC.dat as the lattice geometry file, then the output file will be QMC.den_fmt.

Truncation of Charge Density

If you want to truncate the high coefficient Gs beyond a certain cutoff, you can do this using the keyword using the keyword ke_cutoff which will take a value in Hartrees which corresponds to

$$E_\mathrm{cut} = \frac{1}{2}|\mathbf{G}_\textrm{max}|^2$$

Any Fourier components $\rho_\mathbf{G}$ at a $|\mathbf{G}|\geq|\mathbf{G}_\textrm{max}|$ will be set to zero.

A plot of the Fourier components will be produced which you can disable using the keyword no_write_untrun_g. The reason for doing is mainly to deal with Fourier aliasing from a noisy charge density. A possible metric used in the program is to find out how much the negative part of the charge density integrates to (obviously a physical charge density is positive $\rho(\mathbf{r}) \geq 0$!)

Checking expectation values

This program can also sanity check your CASINO expectation value files from VMC/DMC calculations. This can be done by adding the CHECK_EXPVAL keyword in the .latt file. Doing so does the following:

1. Checks normalisation in reciprocal space (based on zero G-component)
2. Checks for complex conjugate symmetry $\rho_\mathbf{G} = \rho_\mathbf{-G}^*$ (this is REQUIRED since the charge density is real)
3. Checks for inversion symmetry (this is of course only required if it is there to begin with!)

IMPORTANT: In this instance, the primitive lattice vectors will be read from the expval file so you do not need to nor should you specify them in .latt file.

Licence

Copyright (C) 2023 V Ravindran

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.