Gauge invariant coupled cluster response theory

We introduce a time-dependent coupled cluster based Lagrangian that include s orbital rotation. This Lagrangian is shown to give gauge invariant respon se properties for one-electron operators in the limit of a complete one-ele ctron basis. The pole structure of the linear response function is compatib le with that of the exact response function and the notorious problem of un physical second-order poles in the Brueckner coupled cluster response theor y is not present in this model. The total energy of the model is identical to the coupled cluster model using optimized orbitals recently revived by S herrill et al. [J. Chem. Phys. 109, 4171 (1998)]. The model provides a stra ightforward approach for calculating magnetic response properties in a gaug e invariant manner using coupled cluster type wave functions. (C) 1999 Amer ican Institute of Physics. [S0021-9606(99)31317-9].