We have studied the excited electronic states of silylidene(1-silavinylidene), H2CSi, by nb initio quantum-mechanical methods to determine their equilibrium geometries, relative energies, and harmonic vibrational frequencies. Besides the ground (1)A(1) state, we have also studied the (1)A(2) and (3)A(2) states, obtained by exciting an electron from the pi orbital of silylidene to a p(y) lone pair orbital, and the B-1(2) and B-3(2) states obtained by exciting an electron from an a(1) symmetry silicon lone pair orbital to the p(y) orbital. We have also investigated the lowest-lying cation states. Our predictions of the geometries of the (1)A(1) ground state and second excited singlet state, B-1(2),,,, in good agreement with the experimental results of Harper, Waddell, and Clouthier [J. Chem. Phys. 107, 8829 (1997)]. Our predicted excitation energies and excited state vibrational frequencies also agree well with experimental values. The geometry of the (B) over tilde B-1(2) State differs markedly, and unexpectedly, from that of the (X) over tilde(1)A(1) and (B) over tilde(3)B(2) states. We explain this as resulting from the large electron repulsion between the unpaired electrons in the (B) over tilde state, which essentially reside on the same atom. (C) 1997 American Institute of Physics.