The dCSE Project

At the heart of a Castep calculation is the iterative solution of a large eigenvalue problem to find the lowest 1% or so of the eigenstates, called `bands', and this calculation is currently parallelised over the components of the bands. The aim of this project was to implement an additional level of parallelism by distributing the bands themselves. The project was to be comprised of four phases:

1. Basic Band Parallelism
   Split the storage and workload of the dominant parts of a Castep calculation over the `bands', in addition to the current parallelisation scheme.
2. Distributed Matrix Inversion and Diagonalisation
   At various points of a Castep calculation large matrices need to be inverted or diagonalised, and this is currently done in serial. In this phase we will distribute this workload over as many processors as possible.
3. Band-Independent Optimiser
   The current optimisation of the bands requires frequent, expensive orthonormalisation steps that will even more expensive with the new bands-parallelism. Implementing a different, known optimisation algorithm that does not require such frequent orthonormalisation should improve speed and scaling.
4. Parallelisation and Optimisation of New Band Optimiser
   To work on making the new optimiser as fast and robust as possible, and parallelise the new band optimisation algorithm.

The overall aim was to enable Castep to scale efficiently to at least eight times more nodes on HECToR. Unfortunately only the first three phases of the project were funded, but the aim remained to achieve as great an increase in scaling as possible. An additional phase was introduced at the request of NAG, to investigate Castep performance on HECToR in general to determine the best compiler, compiler flags and libraries to use.