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Abstract
Efficient and accurate determination of the effects of electron correlation on chemical systems is the principal task of modern computational quantum chemistry. Experience has shown that ab inito wavefunction methods provide the most reliable means of computating properties of chemical systems. The exponential scaling of the exact electron correlation problem demands the development of approximate solutions that minimize error with respect to computational effort. Herein we discuss the development and utilization of methods for computing the effects of electron correlation on chemical systems. Case studies demonstrating the importance of wavefunction methods and their applicability to experimental results are presented, and new formulas useful for the computation of spectroscopic values are derived. Finally, a new method for the efficient computation of electron correlation in large molecular systems is derived, which represents an archetype for efficient electron correlation computation in the future.