  Coupled cluster (CC) methods are among the most accurate methods in quantum
chemistry. However, the standard CC linear response formulation is not gauge
invariant resulting in errors when modelling properties like optical rotation
and electron circular dichroism. Including an explicit unitary orbital rotation
in the CC Lagrangian makes the linear response function gauge invariant, but
the resulting models are not equivalent to full configuration interaction (FCI)
in the untruncated limit. In this contribution, such methods are briefly
discussed and it is demonstrated that methods using a nonorthogonal orbital
transformation, such as nonorthogonal orbital optimized CC (NOCC), can converge
to FCI in the untruncated limit. This has been disputed in the literature.
