Ida M. B. Nielsen

Accurate structures and binding energies for small water clusters: The water trimer

The global minimum on the water trimer potential energy surface has been investigated by means of second-order Mo/ller-Plesset (MP2) perturbation theory employing the series of correlation-consistent basis sets aug-cc-pVXZ (X = D, T, Q, 5, 6), the largest of which contains 1329 basis functions. Definitive predictions are made for the binding energy and equilibrium structure, and improved values are presented for the harmonic vibrational frequencies. A value of 15.82±0.05u200akcalu200amol−1 is advanced for the infinite basis set frozen core MP2 binding energy, obtained by extrapolation of MP2 correlation energies computed at the aug-cc-pVQZ MP2 geometry. Inclusion of core correlation, using the aug-cc-pCV5Z basis set, has been found to increase the binding energy by 0.08u200akcalu200amol−1, and after consideration of core correlation and higher-order correlation effects, the classical binding energy for the water trimer is estimated to be 15.9±0.2u200akcalu200amol−1. A zero-point vibrational correction of −5.43u200akcalu200amol−1 has bee...

Utilizing High Performance Computing for Chemistry: Parallel Computational Chemistry

Parallel hardware has become readily available to the computational chemistry research community. This perspective will review the current state of parallel computational chemistry software utilizing high-performance parallel computing platforms. Hardware and software trends and their effect on quantum chemistry methodologies, algorithms, and software development will also be discussed.

A new diagnostic for open-shell coupled-cluster theory

Abstract We present a new diagnostic for open-shell coupled-cluster theory, readily computed from the single substitution amplitudes in the CCSD wavefunction. The new diagnostic, D1(ROCCSD), is designed to be comparable to the previously proposed D1(CCSD) diagnostic. Unlike other approaches, the D1 diagnostics are independent of system size and have the same invariance properties as the energy with respect to orbital rotations. Calibration of the D1(ROCCSD) diagnostic on 34 molecular systems indicates that for values of D1(ROCCSD) of 0.025 or below the quality of the CCSD results are, in general, excellent, whereas values larger than 0.025 signal inadequacies in the CCSD approach.

Double-substitution-based diagnostics for coupled-cluster and Møller–Plesset perturbation theory

New diagnostics are proposed for coupled-cluster and Moller–Plesset perturbation theory. The diagnostics, designated D2(CCSD) and D2(MP1), are computed from the double-substitution amplitudes in the CCSD and MP1 wavefunctions, and their definitions are analogous to those of the single-substitution-based D1(CCSD) and D1(MP2) diagnostics. The correlation between the magnitude of the new diagnostics and the performance of the CCSD and MP2 methods is demonstrated for prediction of structures and harmonic vibrational frequencies for 34 molecules, and the application of D1 and D2 to diradicals and bond dissociation is also discussed.

Multi-threading: A new dimension to massively parallel scientific computation

Multi-threading is becoming widely available for Unix-like operating systems, and the application of multi-threading opens new ways for performing parallel computations with greater efficiency. We here briefly discuss the principles of multi-threading and illustrate the application of multi-threading for a massively parallel direct four-index transformation of electron repulsion integrals. Finally, other potential applications of multi-threading in scientific computing are outlined.

Enabling new capabilities and insights from quantum chemistry by using component architectures

Steady performance gains in computing power, as well as improvements in Scientific computing algorithms, are making possible the study of coupled physical phenomena of great extent and complexity. The software required for such studies is also very complex and requires contributions from experts in multiple disciplines. We have investigated the use of the Common Component Architecture (CCA) as a mechanism to tackle some of the resulting software engineering challenges in quantum chemistry, focusing on three specific application areas. In our first application, we have developed interfaces permitting solvers and quantum chemistry packages to be readily exchanged. This enables our quantum chemistry packages to be used with alternative solvers developed by specialists, remedying deficiencies we discovered in the native solvers provided in each of the quantum chemistry packages. The second application involves development of a set of components designed to improve utilization of parallel machines by allowing multiple components to execute concurrently on subsets of the available processors. This was found to give substantial improvements in parallel scalability. Our final application is a set of components permitting different quantum chemistry packages to interchange intermediate data. These components enabled the investigation of promising new methods for obtaining accurate thermochemical data for reactions involving heavy elements.

Multicore challenges and benefits for high performance scientific computing

Until recently, performance gains in processors were achieved largely by improvements in clock speeds and instruction level parallelism. Thus, applications could obtain performance increases with relatively minor changes by upgrading to the latest generation of computing hardware. Currently, however, processor performance improvements are realized by using multicore technology and hardware support for multiple threads within each core, and taking full advantage of this technology to improve the performance of applications requires exposure of extreme levels of software parallelism. We will here discuss the architecture of parallel computers constructed from many multicore chips as well as techniques for managing the complexity of programming such computers, including the hybrid message-passing/multi-threading programming model. We will illustrate these ideas with a hybrid distributed memory matrix multiply and a quantum chemistry algorithm for energy computation using Moller-Plesset perturbation theory.

A novel pseudospectral Fourier method for solving Poisson's equation for a solute in a non-uniform dielectric

A novel pseudospectral method is presented for solving Poissons equation for a solute in a non-uniform dielectric. Poissons equation is transformed to a Helmholtz-like equation which is solved iteratively, as a linear system of equations, by introducing a grid and employing fast Fourier transforms. Several algorithms have been tested for solution of the linear system, and application of an appropriately preconditioned linear solver has been found to provide a robust, rapidly convergent procedure. The method scales nearly linearly with system size, and its implementation is straightforward. Application of the method is demonstrated for computation of the electrostatic contribution to the solvation energy for a wide range of solute molecules, and convergence of the method is shown to be nearly independent of solute and grid size.

Characterization of the sulfur fluoride radical in the ground electronic state

The 2 P ground electronic state of the sulfur fluoride radical has been characterized by high-level ab initio methods, employing the CCSD(T) method with large, augmented correlation-consistent basis sets including aug-cc-pVTZ, augcc-pwCVTZ, and aug-cc-pVQZ. Anharmonic vibrational wave functions have been computed, and previously unavailable transition moments and infrared intensities have been obtained along with dipole moments for the lower vibrational states. A refined theoretical prediction is made for the dipole moment le, and the performance of the CCSD(T) method with the above basis sets is investigated for several other molecular constants for which experimental values are available. 2001 Elsevier Science B.V. All rights reserved.

Scintillating Metal Organic Frameworks: A New Class of Radiation Detection Materials

The detection and identification of subatomic particles is an important scientific problem with implications for medical devices, radiography, biochemical analysis, particle physics, and astrophysics. In addition, the development of efficient detectors of neutrons generated by fissile material is a pressing need for nuclear nonproliferation and counterterrorism efforts. A critical objective in the field of radiation detection is to develop the physical insight necessary to rationally design new scintillation materials for specific applications. However, none of the material types currently used in has sufficient synthetic versatility to exert systematic control over the factors controlling the light output and its dynamics. Here we describe a spectroscopic investigation of two stilbene-based metal-organic frameworks (MOFs) we synthesized, demonstrating that they emit light in response to ionizing radiation, creating the first completely new class of scintillation materials since the advent of plastic scintillators in 1950. This highly novel and unexpected property of MOFs opens a new route to rational design of radiation detection materials, since the spectroscopy shows that both the luminescence spectrum and its timing can be varied by altering the local environment of the chromophore within the MOF. Therefore, the inherent synthetic flexibility of MOFs, which enables both the chromophore structure and its local environment to be systematically varied, suggests that this class of materials can serve as a controlled “nanolaboratory” for probing a broad range of photophysical and radiation detection phenomena. In this presentation we report on the time-dependent fluorescence and radioluminescence of these MOFs and related structures. Multiple decay characteristics have been observed for some materials under study, including fast (ns) exponential and slow (microsecond) non-exponential components. We interpret the results in terms of the electronic states, crystal structures, intermolecular interactions, and transport effects mediating the luminescence. The potential for particle discrimination schemes and large scale production of MOFs and will be discussed.