The wavefunction of a two-electron system without electron-electron repulsion is separable into a simple product of individual hydrogen-like 1s orbitals, making this a foundational textbook fact of quantum mechanics that requires no citation.
The claim states a basic physical principle from elementary quantum mechanics: the Hamiltonian for a two-electron atom (like helium) without electron repulsion terms separates into independent single-particle Hamiltonians, whose solutions are products of 1s wavefunctions. None of the retrieved papers specifically address or test this foundational pedagogical definition; instead, they focus on advanced quantum chemical methods, correlation effects, electron repulsion integrals, and large-scale configuration interaction. Therefore, this is classified as common knowledge by definition.