The ground and the electronically excited states of the C 4 radical are studied using interaction configuration methods and large basis sets. Apart from the known isomers ͓l-C 4 ͑X 3 ⌺ g − ͒ and r -C 4 ͑X 1 A g ͔͒, it is found that the ground singlet surface has two other stationary points:form is the third isomer of this cluster. The isomerization pathways from one form to the other show that deep potential wells are separating each minimum. Multireference configuration interaction studies of the electronic excited states reveal a high density of electronic states of these species in the 0 -2 eV energy ranges. The high rovibrational levels of l-C 4 ͑ 3 ⌺ u − ͒ undergo predissociation processes via spin-orbit interactions with the neighboring 5 ⌺ g + state.

Quartic force fields for the ground electronic states of the most stable C 4 radical isomers [l-C 4 (X 3 AE À g ) and r-C 4 (X 1 A g )] are computed at the same level of theory. These computations are performed using interaction configuration ab initio methods and the cc-pVTZ basis set. These force fields on symmetry-adapted coordinates are derived from full six-dimensional potential energy surfaces generated close to their equilibrium geometries. For both isomers, sets of spectroscopic parameters of the most abundant isotopic varieties are determined with perturbation theory. This includes rotational constants, harmonic wavenumbers, anharmonic wavenumbers for the fundamentals, and some overtones, combination modes, and l-doubling terms. Finally, our results are compared with experimental data measured with different techniques that allow us to discuss assignments of previous astrophysical observations. Specifically, we are focusing in predicting frequencies for the low bendings allowing understanding patterns detected with far-infrared techniques.

We compute the rigid-body, four-dimensional interaction potential between HCO(+) and H2. The ab initio energies are obtained at the coupled-cluster single double triple level of theory, corrected for Basis Set Superposition Errors. The ab initio points are fit onto the spherical basis relevant for quantum scattering. We present elastic and rotationally inelastic coupled channels scattering between low lying rotational levels of HCO(+) and para-/ortho-H2. Results are compared with similar earlier computations with He or isotropic para-H2 as the projectile. Computations agree with earlier pressure broadening measurements.