Roald Hoffmann

Bio: Roald Hoffmann is an academic researcher from Cornell University. The author has contributed to research in topics: Molecular orbital & Electronic structure. The author has an hindex of 116, co-authored 870 publications receiving 59470 citations. Previous affiliations of Roald Hoffmann include Technical University of Denmark & Ajou University.

An Extended Hückel Theory. I. Hydrocarbons

Abstract: The Huckel theory, with an extended basis set consisting of 2s and 2p carbon and 1s hydrogen orbitals, with inclusion of overlap and all interactions, yields a good qualitative solution of most hydrocarbon conformational problems. Calculations have been performed within the same parametrization for nearly all simple saturated and unsaturated compounds, testing a variety of geometries for each. Barriers to internal rotation, ring conformations, and geometrical isomerism are among the topics treated. Consistent σ and π charge distributions and overlap populations are obtained for aromatics and their relative roles discussed. For alkanes and alkenes charge distributions are also presented. Failures include overemphasis on steric factors, which leads to some incorrect isomerization energies; also the failure to predict strain energies. It is stressed that the geometry of a molecule appears to be its most predictable quality.

Orbital interactions in metal dimer complexes

Abstract: A molecular orbital analysis shows that the antiferromagnetic contributions to magnetic coupling, favoring a lowspin ground state for a dimer containing two weakly interacting metal centers, can be analyzed in terms of pairwise interactions of dimeric molecular orbitals, with the square of the splitting in energy between the members of a pair being a measure of the stabilization of the low-spin state. The effect of geometrical distortions, electronegativity, and variation of substituents on the magnetic interaction in dimeric systems is examined in detail for singly bridged L,M-X-ML, ( n = 3 , 4 , 5); Cu~C16~and other doubly bridged species where the bridging ligands are halogens, OR, pyridine N-oxides, oxalate, squarate; and the acetate bridged dimers C u ~ ( R C 0 0 ) 4 . The emphasis is on d9 Cu(I1) dimers, but other transition metal systems are also analyzed. Transition metal complexes containing more than one metal atom with unpaired electrons can generally be categorized according to their magnetic behavior into three main groups depending on the strength of the metal-metal interaction. In the noninteracting type the magnetic properties of the dimer (or polymer) a re essentially unchanged from the paramagnetic monomer. In the strongly interacting type formation of relatively strong metal-metal bonds occurs, and the molecule will display simple diamagnetic behavior (for even numbers of electrons). In this paper the properties of weakly interacting metal ions will be investigated. In such compounds this weak coupling between the electrons of the two metal ions leads to low-lying excited states of different spin which can be populated a t thermal energies (SI000 cm-I). The resulting magnetic behavior will be antiferromagnetic or ferromagnetic, depending on whether the low spin (spins paired) or high spin (spins parallel) state is the ground state, respectively. These interactions-often termed superexchange because of the large distances involved (3-5 A) between the metal ions-have been observed in a wide variety of compounds. I 5 In experimental studies the magnetic interaction between spins SA and Sg for atoms A and B is usually written in a form suggested originally by Heisenberg, Dirac, and Van

Building Bridges Between Inorganic and Organic Chemistry (Nobel Lecture)

Abstract: Robert B. Woodward, a supreme patterner of chaos, was one of my teachers. I dedicate this lecture to him, for it is our collaboration on orbital symmetry conservation, the electronic factors which govern the course of chemical reactions, which is recognized by half of the 1981 Nobel Prize in Chemistry. From Woodward I learned much: the significance of the experimental stimulus to theory, the craft of constructing explanations, and the importance of asethetics in science. I will try to show you how these characteristics of chemical theory may be applied to the construction of conceptual bridges between inorganic and organic chemistry.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

General atomic and molecular electronic structure system

Abstract: A description of the ab initio quantum chemistry package GAMESS is presented. Chemical systems containing atoms through radon can be treated with wave functions ranging from the simplest closed-shell case up to a general MCSCF case, permitting calculations at the necessary level of sophistication. Emphasis is given to novel features of the program. The parallelization strategy used in the RHF, ROHF, UHF, and GVB sections of the program is described, and detailed speecup results are given. Parallel calculations can be run on ordinary workstations as well as dedicated parallel machines. © John Wiley & Sons, Inc.

Chemistry with ADF

Abstract: We present the theoretical and technical foundations of the Amsterdam Density Functional (ADF) program with a survey of the characteristics of the code (numerical integration, density fitting for the Coulomb potential, and STO basis functions). Recent developments enhance the efficiency of ADF (e.g., parallelization, near order-N scaling, QM/MM) and its functionality (e.g., NMR chemical shifts, COSMO solvent effects, ZORA relativistic method, excitation energies, frequency-dependent (hyper)polarizabilities, atomic VDD charges). In the Applications section we discuss the physical model of the electronic structure and the chemical bond, i.e., the Kohn–Sham molecular orbital (MO) theory, and illustrate the power of the Kohn–Sham MO model in conjunction with the ADF-typical fragment approach to quantitatively understand and predict chemical phenomena. We review the “Activation-strain TS interaction” (ATS) model of chemical reactivity as a conceptual framework for understanding how activation barriers of various types of (competing) reaction mechanisms arise and how they may be controlled, for example, in organic chemistry or homogeneous catalysis. Finally, we include a brief discussion of exemplary applications in the field of biochemistry (structure and bonding of DNA) and of time-dependent density functional theory (TDDFT) to indicate how this development further reinforces the ADF tools for the analysis of chemical phenomena. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 931–967, 2001