The (X)over-tilde(1)A(1), (a)over-tilde(3)B(1), (A)over-tilde(1)B(1), and (B)over-tilde(1)A(1) electronic states of SiH2

Four electronically low-lying states of silylene (SiH2) have been studied systematically using high level ab initio electronic structure theory. Self-consistent field (SCF), two-configuration (TC) SCF, complete active space (GAS) SCF, configuration interaction with single and double excitations (CISD), and CASSCF second-order (SO) CI levels of theory were employed with eight distinct basis sets. The zeroth-order wave functions of the ground ((X) over tilde (1)A(1) or 1 (1)A(1)) and (B) over tilde (1)A(1) (or 2 (1)A(1)) excited states are appropriately described by the first and second eigenvectors of the TCSCF secular-equations. The TCSCF-CISD, CASSCF, and CASSCF-SOCI wave functions for the (B) over tilde (1)A(1) (or 2 (1)A(1)) state were obtained by following the second root of the CISD, CASSCF, and SOCI Hamiltonian matrices. At the highest level of theory, the CASSCF-SOCI method with the triple zeta plus triple polarization augmented with two sets of higher angular momentum functions and two sets of diffuse functions basis set [TZ3P (2f, 2d) + 2diff], the energy separation (T-0) between the ground ((X) over tilde (1)A(1)) and first excited ((a) over tilde B-3(1)) states is determined to be 20.5 kcal/mol (0.890 eV, 7180 cm(-1)), which is in excellent agreement with the experimental T-0 value of 21.0 kcal/mol (0.910 eV, 7340 cm(-1)). With the same method the T-0 value for the (A) over tilde B-1(1) - (X) over tilde (1)A(1) separation is predicted to be 45.1 kcal/mol (1.957 eV; 15780 cm(-1)), which is also in fine agreement with the experimental value of 44.4 kcal/mol (1.925 eV, 15530 cm(-1)). The T-0 value for the (B) over tilde (1)A(1) - (X) over tilde (1)A(1) separation is determined to be 79.6 kcal/mol (3.452 eV, 27 840 cm(-1)). After comparison of theoretical and experimental T-0 values for the (a) over tilde B-3(1) and (A) over tilde B-1(1) states and previous studies, error bars for the (B) over tilde (1)A(1) state are estimated to be +/- 1.5 kcal/mol (+/- 525 cm(-1)). The predicted geometry of the (B) over tilde (1)A(1) state is r(e)(SiH) = 1.458 Angstrom and theta(e) = 162.3 degrees. The physical properties including harmonic vibrational frequencies of the (B) over tilde (1)A(1) state are newly determined.