By K. P. Lawley
This sequence goals to record, interpret and overview development within the box of quantum chemistry. Over the process a number of years, the sequence has handled the numerous features of chemical physics and is designed to operate as a vital reference and advisor to extra growth. for every quantity, the editor selects subject matters inside of chemical physics and invitations knowledgeable to put in writing a complete article. quantity sixty nine provides a assessment of contemporary advancements in ab initio equipment in quantum chemistry, protecting the utilized points. experiences from specialists in those parts are integrated.
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Additional info for Advances in Chemical Physics, AB INITIO Methods in Quantum Chemistry - part b (Volume 69)
Therefore. the evaluation of the coupling coefficients is more difficult than for uncontracted wavefunctions, and techniques like GUGA are difficult to apply. Both authors did not treat the more difficult case of semi-internal configurations. A general formulation was first given by Werner and R e i n ~ c h ” . ’ ~ . ‘ ~who , also implemented the method and tested its accuracy. Following the work of Meyers3 they showed that all coupling coefficients can be written as transition density matrices between a set of internal ‘core functions’, which are obtained by applying spin-coupled pair annihilation operators to the reference function.
The resultingcouplingcoefficients involve density matrices up to fifth order, and are more difficult to evaluate. kllL), the matrix elements 48 HANS-JOACHIM WERNER ( Y ; l E i j , k l l Y ; ) are in general not sparse. It is, therefore, unlikely that the contraction of the singly external configurations saves much time, even though there are more non-redundant states YiK than Yfj. This is in line with our experience using this method with a moderate number of reference configurations. F. ' IJ = h V.
So far, two different contraction schemes have been applied. In this case all configurations which differ only in their external part, but not in their internal part, are Contracted. The contraction coefficients are obtained perturbationally. This reduces the number of variational parameters by 2-3 orders of magnitude. The effort in an externally contracted MR-CI calculation is somewhat larger than that in one direct CI iteration for the uncontracted wavefunction. It still depends strongly on the number of reference configurations.
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