Recently, thermally activated delayed fluorescence (TADF) molecules have become the research focus and achieved great progress. Since a narrow energy gap between the singlet-excited state (S) and the triplet-excited state (T) is one important condition to realize TADF, it is necessary to make clear the relationship between geometric structures and the S-T energy gap. In this study, 27 molecules composed of three donors and nine acceptors are investigated based on first-principles calculation. Donors have more significant influence than acceptors on their relative orientations. 9,9-dimethyl-9,10-dihydroacridine and phenoxazine groups are more effective than carbazole in decreasing the S-T energy gap.