Multi-reference coupled cluster theory using the normal ordered exponential ansatz

Properly spin-adapted coupled-cluster theory for general open-shell configurations remains an elusive goal in electronic structure theory. In this contribution we examine Lindgren's normal-ordered exponential ansatz using spin-free excitation operators, with the aid of automatic equation generation software. We present a 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.