Highly correlated ab initio coupled-cluster theories (e.g., CCSD(T), CCSDT) were applied on the ground electronic states Of Si2H3 and Si2H4, with substantive basis sets. A total of 10 isomers, which include mono- and dibridged structures, were investigated. Scalar relativistic corrections and zero-point vibrational energy corrections were included to predict reliable energetics. For Si2H3, we predict an unanticipated monobridged H2Si-H-Si-like structure (C-s, (2)A") to be the lowest energy isomer, in constrast to previous studies which concluded that either H3Si-Si (C-s, (2)A") or near-planar H2Si-SiH (C-1, (2)A) is the global minimum. Our results confirm that the disilene isomer, H2Si-SiH2, is the lowest energy isomer for Si2H4 and that it has a trans-bent structure (C-2h, Ag-1). In addition to the much studied silylsilylene, H3Si-SiH, we also find that a new monobridged isomer H2Si-H-SiH (C-1, (1)A, designated 2c) is a minimum on the potential energy surface and that it has comparable stability; both isomers are predicted to lie about 7 kcal/mol above disilene. By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, the rotational spectrum of this novel Si2H4 isomer has recently been measured in the laboratory, as has that of the planar H2Si-SiH radical. Harmonic vibrational frequencies as well as infrared intensities of all 10 isomers were determined at the cc-pVTZ CCSD(T) level.