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I am new in computational chemistry. I was calculating by "Moller Plesset Second Order perturbation theory -MP2-" (using GAMESS) molecular parameters of Cs

_{3}Cl

_{2}

^{-}. Among them i found two complex frequencies of vibration. My question is now, what is the

**physical meaning**of those complex frequencies?

My aim was actually to evaluate thermodynamic functions of that ion using the rigid-rotator-harmonic oscillator approximation. I am wondering if this approximation still hold for such an ion with a complex frequency,

**is it really a rigid one?**