### Abstract

A quasi-degenerate perturbation method with vibrational self-consistent field (VSCF) reference wavefunction is developed. It simultaneously accounts for strong anharmonic mode-mode coupling among a few states (static correlation) by a configuration interaction theory and for weak coupling with a vast number of the other states (dynamic correlation) by a perturbation theory. A general formula is derived based on the van Vleck perturbation theory. An algorithm that selects a compact set of the most important VSCF configurations which contribute to the static correlation is proposed and a scheme to limit the number of configurations considered for dynamic correlation is also implemented. This method reproduces the vibrational frequencies of CO_{2} and H _{2}CO that are subject to the strongest anharmonic mode-mode coupling within 10 cm^{-1} of vibrational configuration interaction results in a computational expense reduced by a factor of one to two orders of magnitude. The method also reproduces the infrared absorption of C_{6}H_{6} in the CH stretching (ν_{12}) frequency region, in which combination tones ν_{13}ν_{16} and ν_{2}ν _{13}ν_{18} appear on account of an intensity borrowing from ν_{12}via the anharmonic coupling.

Original language | English (US) |
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Pages (from-to) | 1781-1788 |

Number of pages | 8 |

Journal | Physical Chemistry Chemical Physics |

Volume | 10 |

Issue number | 13 |

DOIs | |

State | Published - Mar 28 2008 |

Externally published | Yes |

### ASJC Scopus subject areas

- Physics and Astronomy(all)
- Physical and Theoretical Chemistry

## Fingerprint Dive into the research topics of 'Vibrational quasi-degenerate perturbation theory: Applications to fermi resonance in CO<sub>2</sub>, H<sub>2</sub>CO, and C<sub>6</sub>H<sub>6</sub>'. Together they form a unique fingerprint.

## Cite this

_{2}, H

_{2}CO, and C

_{6}H

_{6}.

*Physical Chemistry Chemical Physics*,

*10*(13), 1781-1788. https://doi.org/10.1039/b719093j