Hello Everyone, Can anyone tell me how to do a transformation from atomic basis to molecular basis incase of 2 electron integrals for the case of Cartesian Gaussian type Orbitals??? Like in case of One electron integrals it is known that a transformation from the Gaussian type orbitals to their contracted form changes the basis from primitive to atomic basis and then doing an Unitary transformation it changes to Molecular Basis, But this is for One electron Integrals?? Any idea 2electron electrons, as Unitary transformation wont work here !!!