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corresponding to a two-center product of 1s-ETOs. For the electrostatic potential of this density, we have

with and . Here, the addition theorem [117], [78]

of the exponential function was used that holds for . Further, the addition theorem (8) of the spherical harmonics was used. Now, the orthonormality of the spherical harmonics yields

The remaining radial integral can be computed with the help of Maple. This approach avoids rounding errors that can easily spoil the calculation of the integral [78]. We note that the result is of the form (17), with

Analogously, one obtains for the multipole expansion of this charge density an equation of the form (9),

and for the multipole moments follows

which can also be computed advantageously with Maple.