HIGH-LEVEL AB-INITIO CALCULATION OF THE ROTATION-VIBRATION ENERGIES IN THE (C)OVER-TILDE (1)A(1) STATE OF METHYLENE, CH2

For the third excited electronic state ((c) over tilde (1)A(1)) of the methylene radical, CH2, we calculate the electronic potential energy surface using a high-level ab initio method and the rotation-vibration energies using a variational technique with a large rotation-vibratio n basis set. The potential energy surface is calculated at a carefully selected grid of 48 nuclear geometries that cover all types of combin ation of stretching and bending deformations to energies more than 20 000 cm(-1) above that of the equilibrium configuration. We fit an anal ytical function, in which we vary 23 parameters, through the points an d find that the state is almost linear with an equilibrium angle of 17 2.7 degrees and a barrier to linearity of only 6 cm(-1). The potential energy points were determined by employing the complete active space self-consistent-field (CASSCF) reference second-order configuration in teraction (SOCI) method. The CASSCF and SOCI wave functions were const ructed following the second eigenvector of the corresponding CI Hamilt onian matrices. It is well-known that theoretical treatments of higher lying states in the same symmetry are substantially tedious and compl icated. The basis set used [TZ3P(2f,2d)+2diff] was triple-zeta plus tr iple polarization with two sets of higher angular momentum functions a nd two sets of diffuse functions. We have used the variational MORBID procedure to calculate the rotation-vibration energies. Because of the peculiar shape of the bending part of the potential surface, some ver y large bending force constants f(0)(i) are obtained, and this has nec essitated the use of very large basis sets in the MORBID calculation i n order to achieve acceptable convergence.