Background

The Discrete-Dipole Approximation (DDA) is a numerical method that represents an arbitrarily-shaped particle as electric dipoles on a cubic lattice. This representation yields a system of linear equations for the induced dipole moments (light-matter interaction). If there are $N$ dipoles representing a particle, this results in a system of linear equations with $3N$ complex-valued unknowns. Typical values of $N$ range between 10,000 and 100,000 so the equations are solved with iterative techniques (i.e. Conjugate Gradient) in high-performance computing clusters. The solutions are used to calculate the theoretical scattering response for any observation direction.

There are many implementations, but the one I used was OpenDDA. In particular, I made some modifications to use the parallelized version using the Message-Passing Interface (MPI) for distributed memory clusters. See below for more details.

Repository Description

This repository contains the modifications I did to the double-precision MPI implementation of OpenDDA, namely:

• Implemented a Corrected Lattice Dispersion Relation in dipole_polarisabilities_MPI.c
• Added a new function scattered_field_MPI.c that calculates the scattered field in terms of 4-by-4 complex-valued matrix, according to the conventions outlined in this article
• Miscellaneous updates to deprecated MPI functions, and modifications to the standard output in the main opendda_MPI.c file

To Compile

1. Download the source code
2. Install the two main library dependencies:

• FFTW
• Open MPI

2. Update the Makefile with path to include and lib folders. The specific location will be installation-dependent
3. Update Makefile with compiler (CC) and relevant flags (CFLAGS)
4. Type make in the terminal to compile the program. The executable is opendda_MPI

To Run

1. Open your terminal and change to the folder containing opendda_MPI
2. Type

mpirun -np X opendda_MPI

where X is the number of processors available

Further Reading

To learn more about Open DDA, see the About OpenDDA file.