Gauge invariance of oscillator strengths in the approximate coupled cluster triples model CC3

The gauge invariance of the oscillator strength is examined for the lowest dipole transitions in Ne, N2 and H2O using the coupled cluster model hierarchy, CCS, CC2, CCSD, and CC3, and a variety of correlation consistent basis sets. The deviations between the oscillator strengths in the different gauges decrease from CCSD to CC3 (where connected triples are included) by about the same factor as they decrease from CCS to CC2 (where connected doubles are included). The deviations are very similar in CC2 and CCSD. The quality of the oscillator strengths is improved significantly at each level in coupled cluster hierarchy, also from CC2 to CCSD, where the equivalence between the oscillator strengths in the different gauges is very similar.