The origin of the emission of the gold phosphine thiolate complex (TPA)AuSCH(CH3)(2) (TPA = 1,3,5-triaza-7-phosphaadamantanetriylphosphine) is investigated using time-dependent density functional theory (TDDFT). This system absorbs light at 3.6 eV, which corresponds mostly to a ligand-to-metal transition with some interligand character. The P-Au-S angle decreases upon relaxation in the S-1 and T-1 states. Our calculations show that these two states are strongly spin-orbit coupled at the ground state geometry. Ligand effects on the optical properties of this complex are also discussed by looking at the simple AuP(CH3)(3)SCH3 complex. The excitation energies differ by several tenths of an electronvolt Excited state optimizations show that the excited singlet and triplet of the (TPA)AuSCH(CH3)(2) complex are bent. On the other hand, the Au-S bond breaks in the excited state for the simple complex, and TDDFT is no longer an adequate method. The excited state energy landscape of gold phosphine thiolate systems is very complex, with several state crossings. This study also shows that the formation of the [(TPA)AuSCH(CH3)(2)](2) dimer is favorable in the ground state. The inclusion of dispersion interactions in the calculations affects the optimized geometries of both ground and excited states. Upon excitation, the formation of a Au-Au bond occurs, which results in an increase in energy of the low energy excited states in comparison to the monomer. The experimentally observed emission of the (TPA)AuSCH(CH3)(2) complex at 1.86 eV cannot be unambiguously assigned and may originate from several excited states.