A method to obtain molecular orbitals specially suited for the variational calculation of the electron-transfer matrix element V-ab is reported. These electron-transfer dedicated molecular orbitals (ET-DMOs) are determined from the diagrammatic second-order expansion of the transfer integral H-ab and are associated to a participation index which allows for a truncation of the molecular orbital basis set and, therefore, for a reduction of the variational space in a difference-dedicated configuration interaction (DDCI) calculation. The suitability of these ET-DMOs is first tested with some simple organic models for which a reduction of the second-order space of 40-60% is possible without significant loss of precision. The calculation of the transfer integral for a Cu(I)-Cu(II) mixed-valence system is also reported. Using as model a cluster of formula Cu2Cl63-, H-ab is estimated to be 2000 cm(-1) from a perturbational-variational calculation. The stability of the ET-DMOs for this system is found to be excellent allowing for a reduction of the variational space of 76% with a relative error of only 0.15%.