Recent Developments in the General Atomic and Molecular Electronic Structure System

A discussion of many of the recently implemented features of GAMESS (General Atomic and Molecular Electronic Structure System) and LibCChem (the C++ CPU/GPU library associated with GAMESS) is presented. These features include fragmentation methods such as the fragment molecular orbital, effective fragment potential and effective fragment molecular orbital methods, hybrid MPI/OpenMP approaches to Hartree-Fock, and resolution of the identity second order perturbation theory. Many new coupled cluster theory methods have been implemented in GAMESS, as have multiple levels of density functional/tight binding theory. The role of accelerators, especially graphical processing units, is discussed in the context of the new features of LibCChem, as it is the associated problem of power consumption as the power of computers increases dramatically. The process by which a complex program suite such as GAMESS is maintained and developed is considered. Future developments are briefly summarized

Developments in fragment-based quantum chemistry: Flattening the curve

The work in this dissertation focuses on the development of novel theoretical models and methods in ab initio fragment-based quantum chemistry. Chapter 2 describes the derivation and application of a mathematically rigorous approach for treating inter-fragment bonds in large covalently bonded systems. Chapter 3 introduces a novel fragment-based quantum chemistry method, referred to as the Expanded Non-Orthogonal Molecular Orbital (ENMO) method, that has been developed by the author. Finally, Chapter 4 describes a fundamental examination on the local nature of the exchange repulsion effect that arises in non-orthogonal formulations of many-particle quantum mechanics as a consequence of the exclusion principle

General, Rigorous Approach for the Treatment of Interfragment Covalent Bonds

A generalized, projection-based transformation of the method-agnostic Fock operator in various ab initio fragment-based quantum chemistry methods has been developed for the treatment of interfragment covalent bonds. This transformation freezes the relevant localized molecular orbital associated with each interfragment bond, thereby restricting the variational subspace of the fragment wave functions, in order to maintain the proper physical characteristics of the involved covalent bonds. In addition, sets of orbitals that would lead to multiple occupancy of certain orbitals are explicitly removed from the variational space. The transformation is developed for the specific case of mutually orthonormal frozen and unfrozen orbitals within each fragment. The newly developed approach is then used to study model systems with two popular ab initio fragment-based methods, and the results of these calculations are compared to those obtained by existing methodologies. Analysis is focused on both quantitative and qualitative accuracy as well as computational scalability and stability. Other methods for which the developed formalisms are appropriate are outlined, and future extensions of the methods are discussed.This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication as Westheimer, Bryce M., and Mark S. Gordon. "General, Rigorous Approach for the Treatment of Interfragment Covalent Bonds." The Journal of Physical Chemistry A 126, no. 39 (2022): 6995-7006. Copyright 2022 American Chemical Society after peer review. To access the final edited and published work see DOI: 10.1021/acs.jpca.2c04015. Posted with permission. DOE Contract Number(s): AC02-07CH11358; ACI-1547580

Scalable ab initio fragmentation methods based on a truncated expansion of the non-orthogonal molecular orbital model

An alternative formulation of the non-orthogonal molecular orbital model of electronic structure theory is developed based on the expansion of the inverse molecular orbital overlap matrix. From this model, a hierarchy of ab initio fragment-based quantum chemistry methods, referred to as the nth-order expanded non-orthogonal molecular orbital methods, are developed using a minimal number of approximations, each of which is frequently employed in intermolecular interaction theory. These novel methods are compared to existing fragment-based quantum chemistry methods, and the implications of those significant differences, where they exist, between the methods developed herein and those already existing methods are examined in detail. Computational complexities and theoretical scaling are also analyzed and discussed. Future extensions for the hierarchy of methods, to account for additional intrafragment and interfragment interactions, are outlined.This article is published as Westheimer, Bryce M., and Mark S. Gordon. "Scalable ab initio fragmentation methods based on a truncated expansion of the non-orthogonal molecular orbital model." The Journal of Chemical Physics 155, no. 15 (2021): 154101. DOI: 10.1063/5.0064864 Copyright 2021 The Author(s). Posted with permission. DOE Contract Number(s): AC02-07CH1133

Core and Uncore Joint Frequency Scaling Strategy

Energy-proportional computing is one of the foremost constraints in the design of next generation exascale systems. These systems must have a very high FLOP-per-watt ratio to be sustainable, which requires tremendous improvements in power efficiency for modern computing systems. This paper focuses on the processor—as still the biggest contributor to the power usage—by considering both its core and uncore power subsystems. The uncore describes those processor functions that are not handled by the core, such as L3 cache and on-chip interconnect, and contributes significantly to the total system power. The uncore frequency scaling (UFS) capability has been available to the user since the Intel Haswell processor generation. In this paper, performance and power models are proposed to use both the UFS and dynamic voltage and frequency scaling (DVFS) to reduce the energy consumption in parallel applications. Then, these models are incorporated into a runtime strategy that performs processor frequency scaling during parallel application execution. The strategy can be implemented at the kernel/firmware level, which makes it suitable for improving the energy efficiency of exascale design. Experiments on a 20-core Haswell-EP machine using the quantum chemistry application GAMESS and NAS benchmark resulted in up to 24% energy savings with as little as 2% performance loss.This article is published as Sundriyal, Vaibhav, Masha Sosonkina, Bryce Westheimer, and Mark Gordon. "Core and uncore joint frequency scaling strategy." Journal of Computer and Communications 6, no. 12 (2018): 184-201. DOI: 10.4236/jcc.2018.612018. Copyright 2022 by authors and Scientific Research Publishing Inc. Attribution 4.0 International (CC BY 4.0). Posted with permission

Recent developments in the general atomic and molecular electronic structure system

A discussion of many of the recently implemented features of GAMESS (General Atomic and Molecular Electronic Structure System) and LibCChem (the C++ CPU/GPU library associated with GAMESS) is presented. These features include fragmentation methods such as the fragment molecular orbital, effective fragment potential and effective fragment molecular orbital methods, hybrid MPI/OpenMP approaches to Hartree–Fock, and resolution of the identity second order perturbation theory. Many new coupled cluster theory methods have been implemented in GAMESS, as have multiple levels of density functional/tight binding theory. The role of accelerators, especially graphical processing units, is discussed in the context of the new features of LibCChem, as it is the associated problem of power consumption as the power of computers increases dramatically. The process by which a complex program suite such as GAMESS is maintained and developed is considered. Future developments are briefly summarized.</p