Ab initio calculations were carried out on cis- and trans-(diphosphino)ethylene digold halide, Au2X2C2H2(PH2)(2) (X = Cl, Br, I), the model compounds for dinuclear gold(I) bis(diphenylphosphino)ethylene complexes, Au2X2(dppee). The calculations reveal aurophilic interactions in the cis compound, thus stabilizing the cis complex with respect to the trans species. The aurophilic interaction, caused by an interplay between relativistic and electron correlation effects, increases with increasing softness of the ligand attached to gold. Excited-state calculations show ethylene pi* contributions only for the cis compound, but not for the trans compound, thus explaining why the trans complex does not isomerize to the cis compound upon UV radiation at wavelengths greater than 220 nm. The singlet excitation can be best described as a charge-transfer transition from the halogen lone pair to the Au (5d6s6p) orbitals with strong intermixing from phosphorus (sp) orbitals. Because of symmetry reasons, these orbitals mix with the ethylene C=C pi* orbital for the cis compound. As a result, the aurophilic interaction causes a stabilization of the first excited singlet state. This causes a red shift in the spectrum of the cis isomers relative to the trans isomers, which increases with decreasing Au-Au bond distance. The cis compound also shows enhanced transition probabilities compared to the trans compound for the first electronic singlet excitation. Nonrelativistic calculations reveal only a weak bond between the C2H2(PH2)(2) unit and Au2Cl2. The calculated Au-Au distances in Au2X2 decrease from X = F down the group to X = I. This contrasts with the increasing Au-X bond distance when going to the heavier halides and indicates an aurophilic attraction.