The coupled‐cluster method with a multiconfiguration reference state

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Abstract
The coupled‐cluster approach to obtaining the bond‐state wave functions of many‐electron systems is extended, with a set of physically reasonable approximations, to admit a multiconfiguration reference state. This extension permits electronic structure calculations to be performed on correlated closed‐ or open‐shell systems with potentially uniform precision for all molecular geometries. Explicit coupled cluster working equations are derived using a multiconfiguration reference state for the case in which the so‐called cluster operator is approximated by its one‐ and two‐particle components. The evaluation of the requisite matrix elements is facilitated by use of the unitary group generators which have recently received wide attention and use in the quantum chemistry community.

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