Abstract
The two-electron problem is investigated using exponentially parametrized wave functions for various different coupled-cluster (CC) methods, including regular, expectation value, symmetrized expectation value (i.e., unitary), extended, and quadratic configuration interaction (CI) variants. All are viewed as arising from alternative energy functionals. This pedagogical evaluation demonstrates the differences in these methods, including the role of EPV terms. © 1992 John Wiley & Sons, Inc.
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