Advances in the Parallelisation of Software for Quantum Chemistry Applications

Density functional theory (DFT) provides some of the most important methods used in computational theory today. They allow one to determine the electronic structure of finite chemical systems, be they molecules or clusters, using a quantum-mechanical model, and exposes, thus, the great majority of the systems’ properties relevant to chemical applications. However, the numerical treatment of large chemical systems proves to be expensive, requiring elaborate parallelisation strategies.This paper presents two recent developments which aim at improving the parallel scalability of the quantum chemistry code ParaGauss. First, we introduce a new Fortran interface to parallel matrix algebra and its library implementation. This interface specifies a set of distributed data objects, combined with a set of linear algebra operators. Thus, complicated algebraic expressions can be expressed efficiently in pseudo-mathematical notation, while the numerical computations are carried out by back-end parallel routines. This technique is evaluated on relativistic transformations, as implemented in ParaGauss.The second development addresses the solution of the generalized matrix eigenvalue problem—an inherent step in electronic structure calculations. In the case the symmetry of a molecule is exploited, pertinent matrices expose a block-diagonal structure which makes the efficient use of existing parallel eigenvalue solvers difficult. We discuss a technique that uses a malleable parallel task scheduling (MPTS) algorithm for scheduling instances of parallel ScaLAPACK-routines on the available processor resources. This technique significantly improves the parallel performance of this numerical step, reducing the corresponding execution time to below 1 s in most applications considered.