Orbital molecular, gauge-dependent

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. 

Modified, gauge-dependent (localised) molecular orbitals are introduced through... 

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