Hello, I am new in computational chemistry. I was calculating by "Moller Plesset Second Order perturbation theory -MP2-" (using GAMESS) molecular parameters of Cs3Cl2-. 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?