Gauge-including atomic orbital basis sets

As far as the most appropriate computational methodology to compute nuclear chemical shielding and NICS values is concerned, despite the considerable debate in the literature, the gauge including atomic orbital (GIAO) scheme was widely regarded as the most accurate when a large basis set is used [59]. 

Note The results presented here have been calculated using the gauge-including atomic orbitals (GIAO) method at MP2/6-311 +G(d) and B3LYP/6-311 +G(d) (values presented within parentheses) level. 6-31 -l- G(d) basis set in case of cluster contains the Ga atom. 

Another difficulty arises from the necessity of summing over all the excited states of the molecule, including the continuum. In general little is known about such states for most molecules. In addition, Ramsey s formulation produces screening data which depend upon the choice of origin, i.e. are gauge dependent, unless a complete basis set of atomic orbitals is included in the molecular orbital description. This is rarely possible, even for diatomic molecules, without the use of large amounts of computer time. 

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