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 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 about 2electron integrals, as Unitary transformation wont work here !!!