A high-level ab initio study of the N2 + N2 reaction channel

A new six-dimensional (6D) global potential energy surface (PES) is proposed for the full range description of the interaction of the N2(1Σg+)+N2(1Σg+) system governing collisional processes, including N atom exchange. The related potential energy values were determined using high-level ab initio methods. The calculations were performed at a coupled-cluster with single and double and perturbative triple excitations level of theory in order to have a first full range picture of the PES. Subsequently, in order to accurately describe the stretching of the bonds of the two interacting N2 molecules by releasing the constraints of being considered as rigid rotors, for the same molecular geometries higher level of theory multi reference calculations were performed. Out of the calculated values a 6D 4-atoms global PES was produced for use in dynamical calculations. The ab initio calculations were made possible by the combined use of High Throughput Computing and High Performance Computing techniques within the frame of a computing grid empowered molecular simulator.

Ab initio Variational Transition State Theory and Master Equation Study of the Reaction (OH)3SiOCH2 + CH3 ⇌ (OH)3SiOC2H5

In this paper we use variable reaction coordinate variational transition state theory (VRC-TST) to calculate the reaction rate constants for the two reactions, R1: (OH)3SiOCH2 + CH3 ⇌ (OH)3SiOC2H5, and R2: CH2OH + CH3 ⇌ C2H5OH. The first reaction is an important channel during the thermal decomposition of tetraethoxysilane (TEOS), and its rate coefficient is the main focus of this work. The second reaction is analogous to the first and is used as a basis for comparison. The interaction energies are obtained on-the-fly at the CASPT2(2e,2o)/cc-pVDZ level of theory. A one-dimensional correction to the sampled energies was introduced to account for the energetic effects of geometry relaxation along the reaction path. The computed, high-pressure rate coefficients were calculated to be, R1: k 1 = 2.406 × 10−10 T −0.301 exp (− 271.4/T) cm3 molecule –1 s –1 and R2: k 2 = 1.316 × 10−10 T −0.189 exp (− 256.5/T) cm3 molecule –1 s –1. These rates differ from each other by only 10% – 30% over the temperature range 300–2000 K. A comparison of the computed rates with experimental data shows good agreement and an improvement over previous results. The pressure dependency of the reaction R1 is explored by solving a master equation using helium as a bath gas. The results obtained show that the reaction is only weakly pressure dependent over the temperature range 300–1700 K, with the predicted rate constant being within 50% of its high-pressure limit at atmospheric pressure.

Introduction to High-Performance Computing

Since the first general-purpose computing machines came up in the middle of the twentieth century, computer science's popularity has been growing steadily until our time. The first computers represented a significant leap forward in automating calculations, so that several theoretical methods could be taken from paper into practice. The continuous need for increased computing capacity made computers evolve and become more and more powerful. Nowadays, high-performance computing (HPC) is a crucial component of scientific and technological advancement. This book chapter introduces the field of HPC, covering key concepts and essential terminology to understand this complex and rapidly evolving area. The chapter begins with an overview of what HPC is and how it differs from conventional computing. It then explores the various components and configurations of supercomputers, including shared memory systems, distributed memory systems, and hybrid systems and the different programming models used in HPC, including message passing, shared memory, and data parallelism. Finally, the chapter discusses significant challenges and future directions in supercomputing. Overall, this chapter provides a comprehensive introduction to the world of HPC and is an essential resource for anyone interested in this fascinating field.