Density fitting (also known as the resolution of the identity) is a standard computational technique in quantum chemistry that approximates four-center two-electron integrals by expanding the charge distribution over an auxiliary basis, effectively reducing them to combinations of three-center and two-center integrals. No citation is needed for this fundamental definition.
Density fitting approximates four-center integrals by expanding the charge distribution, reducing them to combinations of three-center and two-center integrals.
The claim describes a standard, definitional aspect of density fitting / resolution of the identity in quantum chemistry where four-center integrals are reduced via auxiliary basis sets. Per instructions, fundamental definitions or direct descriptions of well-known methods are treated as common knowledge when they are trivially true by definition.