If one reflects upon the range of chemical problems accessible to the current quantum theoretical methods for calculations on the electronic structure of molecules, one is immediately struck by the rather narrow limits imposed by economic and numerical feasibility. Most of the systems with which experimental photochemists actually work are beyond the grasp of ab initio methods due to the presence of a few reasonably large aromatic ring systems. Potential energy surfaces for all but the smallest molecules are extremely expensive to produce, even over a restricted group of the possible degrees of freedom, and molecules containing the higher elements of the periodic table remain virtually untouched due to the large numbers of electrons involved. Almost the entire class of molecules of real biological interest is simply out of the question. In general, the theoretician is reduced to model systems of variable appositeness in most of these fields. The fundamental problem, from a basic computational point of view, is that large molecules require large numbers of basis functions, whether Slater­ type orbitals or Gaussian functions suitably contracted, to provide even a modestly accurate description of the molecular electronic environment. This leads to the necessity of dealing with very large matrices and numbers of integrals within the Hartree-Fock approximation and quickly becomes both numerically difficult and uneconomic.

1. Ground-State Potential Surfaces and Thermochemistry.- 1. Introduction.- 2. Macroscopic Properties from Molecular Calculations.- 3. Semiempirical Molecular Orbital Theory for Closed Shells.- 4. Exploring Potential Energy Surfaces.- 5. Selected Results and Comparisons.- 6. Conclusions and an Opinion.- References.- 2. Electronic Excited States of Organic Molecules.- 1. Introduction.- 2. The Hamiltonian Operator.- 3. The Zeroth-Order Approximation.- 4. The Electronic Wave Function.- 5. The Interaction of Matter and Electromagnetic Fields.- 6. Spin-Orbit and Spin-Spin Coupling.- 7. Vibrationally Induced Transitions.- 8. Application of ZDO Methods.- 9. Conclusions.- References.- 3. Photochemistry Josef Michl.- 1. Introduction.- 2. Photochemical Processes.- 3. Semiempirical Methods.- 4. Examples of Application.- 5. Summary and Outlook.- References.- 4. Approximate Methods for the Electronic Structures of Inorganic Complexes.- 1. Inorganic Complexes Contrasted to Organic Molecules.- 2. TheOrbitals.- 3. The Ligand Field and the Crystal Field Methods.- 4. Koopmans' Theorem.- 5. Spin-Orbit Coupling.- 6. NonempiricalCNDO and INDO Methods.- 7. Semiempirical CNDO and INDO Methods.- 8. The Excited States.- 9. The Crystal Field Theory.- 10. Extended Hückel Theory. Angular Overlap Model.- 11. An Example, Ni(CN)4~. Conclusions.- References.- 5. Approximate Molecular Orbital Theory of Nuclear and Electron Magnetic Resonance Parameters.- 1. Introduction.- 2. Magnetic Resonance Parameters.- 3. Molecular Quantum Mechanics.- 4. NMR Shielding Constants and Chemical Shifts.- 5. NMR Nuclear Spin Coupling Constants.- 6. ESRg-Tensors.- 7. ESR Electron-Nuclear Hyperfine Tensors.- 6. The Molecular Cluster Approach to Some Solid-State Problems.- 1. Introduction.- 2. Solid-State TheoryApproaches to Surface Problems.- 3. Molecular Cluster Approach to Surface Problems.- 4. Summary.- References.- 7. Electron Scattering Donald G. Truhlar.- 1. Introduction.- 2. Explicit Inclusion of Electronic Excitations.- 3. Neglect of Electronic Excitation Except for Final State.- 4. Inclusion of Effect of Omitted Electronic States by Approximate Polarization Potentials.- References.- Author Index.- Molecule Index.