CASSCF (Complete Active Space Self Consistent Field) - Wave Packet AB initio Prediction of Electronic and Vibrational Spectra: Application to the (A(to the Second Power Pi) reversible reaction X (to the Second Power Sigma (+))) Absorption of C(2)H at 3000K.

摘要

The combination of ab initio calculation of the electronic wavefunctions with a wave packet calculation of the nuclear motion is used, within the Born-Oppenheimer approximation, to compute the vibrational and electronic absorption of a polyatomic molecule. A particular virtue of this approach is that high as well as low temperature spectra are both calculable. This method is applied to C2H, for which the Complete Active Space Self Consistent Field (CASSCF) method is used to determine full Born Oppenheimer potential surfaces. Using the assumption that the (A(to the 2nd power Pi) reversible reaction X(to the 2nd power sigma (+)) absorption can be written as the sum of the A(to the 2nd power A prime) reversible reaction X) and A(to the second power A double prime) reversible reaction X) absorptions, the spectra are determined to 60/cm resolution at a temperature of 3000K. As a result of the large thermal bending amplitude at 3000K, the calculated spectra are broad and have little resolved structure. Two bands are resolvable, one is due to the A(to the 2nd power A double prime) reversible reaction X) absorption and is centered at 5500/cm, while the other is due to the A(to the 2nd power A prime) reversible reaction X) absorption and is centered at 9500/cm. The dramatic blue shift of the A(to the 2nd power A prime) reversible reaction X) band results from the combination of the large X state thermal bending amplitude and the high bending frequency of the A(to the 2nd power A double prime) state. We also determine the X state pure vibrational absorption spectrum and show it to be of much lower intensity than the pure electronic spectrum.