The development of reliable tools for excited-state simulations is very important for understanding complex processes in the broad class of light harvesting systems and optoelectronic devices. Over the last years we have been developing equation of motion coupled cluster (EOMCC) methods capable of tackling these problems. In this paper we discuss the parallel performance of EOMCC codes which provide accurate description of excited-state correlation effects. Two aspects are discussed in detail: (1) a new algorithm for the iterative EOMCC methods based on improved parallel task scheduling algorithms, and (2) parallel algorithms for the non-iterative methods describing the effect of triply excited configurations. We demonstrate that the most computationally intensive non-iterative part can take advantage of 210,000 cores of the Cray XT5 system at the Oak Ridge Leadership Computing Facility (OLCF), achieving over 80% parallel efficiency. In particular, we demonstrate the importance of the computationally demanding non-iterative many-body methods in matching experimental level of accuracy for several porphyrin-based systems.

An important advance in cluster computing is the evolution from single processor clusters to multi-processor SMP clusters. Due to the increased complexity in the memory model on SMP clusters, new approaches are needed for applications that make use of distributed-memory paradigms. This paper presents new communications software developments that are designed to take advantage of SMP cluster hardware. Although the specific focus is on the central field of computational chemistry and materials science, as embodied in the popular electronic structure package GAMESS (General Atomic and Molecular Electronic Structure System), the impact of these new developments will be far broader in scope. Following a summary of the essential features of the distributed data interface (DDI) in the current implementation of GAMESS, the new developments for SMP clusters are described. The advantages of these new features are illustrated using timing benchmarks on several hardware platforms, using a typical computational chemistry application.