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Parallel computing in quantum chemistry -- Message passing and beyond for a general ab initio program system
One of the most prominent aims in Computational Chemistry is the modeling of chemical reactions and the prediction of molecular properties. Quantum chemical methods are used for the calculation of molecular structures, spectra, reaction energy profiles and many other interesting quantities. Nowadays, the accuracy of the theoretical calculations can compete to an increasing extent with the experimental one. A great variety of quantum chemical methods exist ranging from the standard Hartree-Fock theory to sophisticated electron correlation approaches. From a computational point of view all these methods require rather lengthy and complicated program codes and have to handle a large amount of data to be stored on external devices. In the simplest case, the Hartree-Fock (SCF) method, ``direct`` algorithms have eliminated the I/O and storage bottleneck and have opened the way to parallel implementations. For post-Hartree-Fock methods the situation is much more complicated as will be demonstrated below. Therefore, most of the previous attempts in parallelizing quantum chemical ab initio programs concentrated on SCF methods. The authors investigations presented here are a continuation of their previous work on the parallelization of the COLUMBUS program system. The COLUMBUS program is based on the multireference single- and double-excitation configuration interaction (MRSDCI) approach. It is very portable and runs on a large variety of computers including numerous Unix-based workstations, VAX/VMS minicomputers, IBM mainframes and Cray supercomputers
Minimalistic Descriptions of Nondynamical Electron Correlation: From Bond-Breaking to Transition-Metal Catalysis
From a theoretical standpoint, the accurate description of potential energy surfaces for bond breaking and the equilibrium structures of metal-ligand catalysts are distinctly similar problems. Near degeneracies of the bonding and anti-bonding orbitals for the case of bond breaking and of the partially-filled d-orbitals for the case of metal-ligand catalyst systems lead to strong non-dynamical correlation effects. Standard methods of electronic structure theory, as a consequence of the single-reference approximation, are incapable of accurately describing the electronic structure of these seemingly different theoretical problems. The work within highlights the application of multi-reference methods, methods capable of accurately treating these near-degeneracies, for describing the bond-breaking potentials in several small molecular systems and the equilibrium structures of metal-salen catalysts. The central theme of this work is the ability of small, compact reference functions for accurately describing the strong non-dynamical correlation effects in these systems.Ph.D.Committee Chair: C. David Sherrill; Committee Member: Jean-Luc Bredas; Committee Member: Mostafa El-Sayed; Committee Member: Peter J. Ludovice; Committee Member: Thomas Orland