The rovibrational energy levels of quasilinear (c)over-tilde(1)A(1) methylene

Using ab initio methods we have calculated the potential energy surface of the third excited electronic state ((c) over tilde(1)A(1)) of the methylene radical CH2 at a grid of 51 nuclear geometries which probe energies more than 20 000 cm(-1) above the equilibrium energy of the state. The method employed the second eigenvalue of a secular equation incorporating all single and double excitations relative to a two-configuration self-consistent-field reference function. The basis set used [TZ3P(2f,2d) + 2diff] was triple zeta plus triple polarization with two sets of higher angular momentum functions and two sets of diffuse functions. We find the state to be almost linear with an equilibrium angle of 171.6 degrees, a barrier to linearity of only 12 cm(-1), and an electronic term value T-e((c) over tilde) of 21 080 cm(-1). After fitting an analytical function through the points we have used the variational MORBID procedure to calculate the rotation-vibration energies; we tabulate the calculated energies. This state is of interest both to theoreticians, because it presents the challenge of not being the lowest state of its symmetry, and to experimentalists, because its spectral signature has not yet been characterized. Its near linearity remains to be proven by experiment or perhaps by a higher level of ab initio theory. (C) 1996 Academic Press, Inc.