The performance of the IQA energy partition coupled with approximate two-particle density matrices (2PDMs) was studied. BBC1, BBC2, Muller, and CGA density matrix functionals (DMFs) were used to derive approximate 2PDMs for Cu-2, CuH, and (CuH)(2). All DMFs used result in significant errors in molecular energies compared with those evaluated using exact response CCSD 2PDMs. Using the DMFs to derive approximate 2PDMs leads to noticeably smaller errors in energies of interatomic interactions in the model species and in CuCu binding energies for (CuH)(2). Muller and CGA DMFs are most suitable for IQA evaluation of the strength of CuCu interactions.