title: Fermi resonance in CO2: a combined electronic coupled-cluster and vibrational configuration-interaction prediction.
abstract: The authors present a first-principles prediction of the energies of the eight lowest-lying anharmonic vibrational states of CO(2), including the fundamental symmetric stretching mode and the first overtone of the fundamental bending mode, which undergo a strong coupling known as Fermi resonance. They employ coupled-cluster singles, doubles, and (perturbative) triples [CCSD(T) and CCSDT] in conjunction with a range of Gaussian basis sets (up to cc-pV5Z, aug-cc-pVQZ, and aug-cc-pCVTZ) to calculate the potential energy surfaces (PESs) of the molecule, with the errors arising from the finite basis-set sizes eliminated by extrapolation. The resulting vibrational many-body problem is solved by the vibrational self-consistent-field and vibrational configuration-interaction (VCI) methods with the PESs represented by a fourth-order Taylor expansion or by numerical values on a Gauss-Hermite quadrature grid. With the VCI, the best theoretical estimates of the anharmonic energy levels agree excellently with experimental values within 3.5 cm(-1) (the mean absolute deviation). The theoretical (experimental) anharmonic frequencies of the Fermi doublet are 1288.9 (1285.4) and 1389.3 (1388.2) cm(-1).
authors.affiliation: Quantum Theory Project, Department of Chemistry, University of Florida, Gainesville, Florida 32611-8435.
authors.forename: Valerie
authors.initials: V
authors.lastname: Rodriguez-Garcia
authors.forename: So
authors.initials: S
authors.lastname: Hirata
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Yagi
authors.forename: Kimihiko
authors.initials: K
authors.lastname: Hirao
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Taketsugu
authors.forename: Igor
authors.initials: I
authors.lastname: Schweigert
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Tasumi