Highly accurate calculations are reported for properties of vinylidene (H2C=C:), specifically the position of its zero-point vibrational level relative to that of acetylene and its equilibrium structure and ground state rotational constants. The isomerization energy of vinylidene calculated at the HEAT-456QP level of theory is 43.53 +/- 0.15 kcal mol(-1), in agreement with the previous best estimate, but associated with a much smaller uncertainty. In addition, the thermochemical calculations presented here also allow a determination of the H2CC H bond energy of the vinyl radical at the HEAT-345(Q) level of theory, which is 77.7 +/- 0.3 kcal mol(-1). The equilibrium structure of vinylidene, estimated with an additivity scheme that includes treatment of correlation effects beyond CCSD(T) as well as relativistic and adiabatic (diagonal Born-Oppenheimer correction) contributions, is r(CC) = 1.2982 +/- 0.0003 angstrom, r(CH) = 1.0844 +/- 0.0003 A, and theta(CCH) = 120.05 +/- 0.05 degrees, with zero-point rotational constants (including vibrational contributions and electronic contributions to the moment of inertia) estimated to be A(0) = 9.4925 +/- 0.0150 cm(-1), B-0 = 1.3217 +/- 0.0017 cm(-1), and C-0 = 1.1602 +/- 0.0016 cm(-1).