Martyn F. Guest

QUASI: A general purpose implementation of the QM/MM approach and its application to problems in catalysis

Abstract We describe the work of the European project QUASI (Quantum Simulation in Industry, project EP25047) which has sought to develop a flexible QM/MM scheme and to apply it to a range of industrial problems. A number of QM/MM approaches were implemented within the computational chemistry scripting system, ChemShell, which provides the framework for deploying a variety of independent program packages. This software was applied in several large-scale QM/MM studies which addressed the catalytic decomposition of N 2 O by Cu-containing zeolites, the methanol synthesis reaction catalysed by Cu clusters supported on ZnO surfaces, and the modelling of enzyme structure and reactivity.

The GAMESS-UK electronic structure package: algorithms, developments and applications

A description of the ab initio quantum chemistry package GAMESS-UK is presented. The package offers a wide range of quantum mechanical wavefunctions, capable of treating systems ranging from closed-shell molecules through to the species involved in complex reaction mechanisms. The availability of a wide variety of correlation methods provides the necessary functionality to tackle a number of chemically important tasks, ranging from geometry optimization and transition-state location to the treatment of solvation effects and the prediction of excited state spectra. With the availability of relativistic ECPs and the development of ZORA, such calculations may be performed on the entire Periodic Table, including the lanthanides. Emphasis is given to the DFT module, which has been extensively developed in recent years, and a number of other, novel features of the program. The parallelization strategy used in the program is outlined, and detailed speedup results are given. Applications of the code in the areas of enzyme and zeolite catalysis and in spectroscopy are described.

The electronic states of furan studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction calculations

Abstract The results of large-scale ab initio CI calculations on the electronic states of furan, using a multi-reference multi-root CI method (MRD-CI), are related to new VUV absorption and low-energy electron energy-loss (EEL) measurements, leading to detailed spectral assignments. All of the expected 1,3 ππ ∗ states have been assigned, there being improved agreement between theoretical and experimental energies in all cases. A number of new Rydberg series has been identified by experiment and, for these also, the calculations perform well. In addition, cationic states and ground state molecular electronic properties have been computed.

An investigation of self-consistent field perturbation theory applied to the calculation of nuclear spin-spin coupling constants

An initio SCF perturbation theory in the formulation of Ditchfield and Snyder has been applied to the calculation of the N.M.R. spin-spin coupling constants in CH4, NH3, H2O and HF using large gaussian basis sets, and including all second-order contributions (Fermi contact, orbital and dipolar). In accordance with most other work, the first-order term in the orbital contribution has been neglected, however. The sensitivity of the coupling constants to variations in the basis set has been studied, and the reliability of the method is discussed in the light of these results. It is shown that the Fermi contact contribution provides the largest component of the calculated coupling constants, with the dipolar term being generally negligible. However, the orbital contribution is found to be non-negligible, particularly in the case of the directly bonded couplings in HF and H2O, and is shown to be the primary reason, at the level of calculation adopted, for variations in the HH geminal couplings in the series CH...

Parallelism in computational chemistry: I. Hypercube-connected multicomputers

SummaryAn account is given of experience gained in implementing computational chemistry application software, including quantum chemistry and macromolecular refinement codes, on distributed memory parallel processors. In quantum chemistry we consider the coarse-grained implementation of Gaussian integral and derivative integral evaluation, the direct-SCF computation of an uncorrelated wavefunction, the 4-index transformation of two-electron integrals and the direct-CI calculation of correlated wavefunctions. In the refinement of macromolecular conformations, we describe domain decomposition techniques used in implementing general purpose molecular mechanics, molecular dynamics and free energy perturbation calculations. Attention is focused on performance figures obtained on the Intel iPSC/2 and iPSC/860 hypercubes, which are compared with those obtained on a Cray Y-MP/464 and Convex C-220 minisupercomputer. From this data we deduce the cost effectiveness of parallel processors in the field of computational chemistry.

Abinitio predictions of structural and optical response properties of Na+n clusters: Interpretation of depletion spectra at low temperature

We show that a comparison of the depletion spectra of Na+n (n=2–9,11,21) clusters recorded at low temperature and optically allowed transitions determined for the stable structures using ab initio methods accounting for electron correlation allows the assignment of the cluster geometry to the measured features. Due to the large mobility of atoms in alkali metal clusters, the influence of temperature on structural and electronic properties is significant. The lowering of temperature reveals new spectroscopic features which are structure dependent. Optical response properties of small cationic Na+n clusters are characterized by rich molecularlike spectroscopic patterns, also with increasing size, and differ substantially from those found for neutral clusters. It has been clearly demonstrated that not only the number of valence electrons but its mutual interplay with the geometric properties determine optical response features.

Analytical potentials for triatomic molecules from spectroscopic data: V. Application to HOX (X=F, Cl, Br, I)

Analytical potential energy functions are reported for HOX (X=F, Cl, Br, I). The surface for HOF predicts two metastable minima as well as the equilibrium configuration. These correspond to HFO (bent) and OHF (linear). Ab initio calculations performed for the HOF surface confirm these predictions. Comparisons are drawn between the two sets of results, and a vibrational analysis is undertaken for the hydrogen bonded OHF species. For HOCl, one further minimum is predicted, corresponding to HClO (bent), the parameters for which compare favourably with those reported from ab initio studies. In contrast, only the equilibrium configurations are predicted to be stable for HOBr and HOI.

Toward high‐performance computational chemistry: II. A scalable self‐consistent field program

We discuss issues in developing scalable parallel algorithms and focus on the distribution, as opposed to the replication, of key data structures. Replication of large data structures limits the maximum calculation size by imposing a low ratio of processors to memory. Only applications which distribute both data and computation across processors are truly scalable. The use of shared data structures that may be independently accessed by each process even in a distributed memory environment greatly simplifies development and provides a significant performance enhancement. We describe tools we have developed to support this programming paradigm. These tools are used to develop a highly efficient and scalable algorithm to perform self‐consistent field calculations on molecular systems. A simple and classical strip‐mining algorithm suffices to achieve an efficient and scalable Fock matrix construction in which all matrices are fully distributed. By strip mining over atoms, we also exploit all available sparsity and pave the way to adopting more sophisticated methods for summation of the Coulomb and exchange interactions.

Excited states of HCl

Ab initio CI calculations are presented for potential curves for the valence states of HCl, for the vertical electronic spectrum and for the potential curve for the B 1Σ+ state. The valence 1Π, 3Π and 3Σ+ states are found to be repulsive. Calculated vertical excitation energies are in good agreement with experiment. The B 1Σ+ curve is predicted to have a double minimum.

Triple and quadruple excitations and the valence correlation energies of small molecules

Abstract The contribution of triply-excited states and quadruply-excited states to the valence correlation energy is investigated for a number of atoms and molecules using the diagrammatic many-body perturbation theory. The importance of triple excitations in accurate calculations is demonstrated.