Title full: Full valence complete active space SCF, multireference CI, and density functional calculations of 1A1-3B1 singlet-triplet gaps for the valence-isoelectronic series BH-2, CH2, NH+2, AlH-2, SiH2, PH+2, GaH-2, GeH2, and AsH+2. MRCISD(Q)/CASSCF calculations have been performed for the title dihydrides. The agreement with available experimental data is quite good for the 1A1-3B1 singlet-triplet (S-T) energy gaps, as is agreement with other calculations of similar quality. DFT calculations employing basis sets of similar quality with non-local exchange and correlation corrections are two to three orders of magnitude faster than MRCI. DFT agrees well with MRCI for the first and third rows, although the use of different correlation functionals is required. Results in the second row are only fair with either correlation functional.