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Multi-reference coupled cluster theory using the normal ordered exponential ansatz
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DOI：10.1039/d4fd00044g.png)
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Abstract
En 中文
Properly spin-adapted coupled-cluster theory for general open-shell configurations remains an active area of research in electronic structure theory. In this contribution we examine Lindgren's normal-ordered exponential ansatz to correlate specific spin states using spin-free excitation operators, with the aid of automatic equation generation software. We present an intermediately normalised and size-extensive reformulation of the unlinked working equations, and analyse the performance of the method with single and double excitations for simple molecular systems in terms of accuracy and size-consistency. We examine Lindgren's normal-ordered exponential ansatz to correlate specific spin states using spin-free excitation operators, with the aid of automatic equation generation software.
Keywords:
CONNECTED TRIEXCITED CLUSTERS
EXCITED-STATES
CONVERGENCE