Semiclassical calculation of the vibrational structure of theBtilde;thinsp;1BuRydberg state oftranshyphen;dihyphen;imide fromabinitioconfiguration interaction potential energy surfaces

摘要

The applicability of the SDCI and CEPA methods to the excited state of the title molecule is investigated. Two basis sets are used, one of triple zeta quality extended with diffuse functions and another which also contained polarization functions. For the subsequent CI calculations closed shell as well as open shell, molecular orbitals are used. We investigate the Pople correction as a way to obtain size consistent results from the SDCI calculation. For each method, a twohyphen;dimensional potential energy surface of the1(4ag,5bu) Rydberg state oftranshyphen;dihyphen;imide (HNNH) is calculated. The vibrational fine structure in the correspondingBtilde;larr;Xtilde; UV absorption spectrum is derived from these surfaces and the result is compared to the spectrum measured by Neudorfletal. lsqb;P. S. Neudorfl, R. A. Back, and A. E. Douglas, Can. J. Chem.59, 506 (1981)rsqb;. A semiclassical method lsqb;K. S. Sorbie, Mol. Phys.32, 1577 (1976)rsqb; is used to obtain the vibrational frequencies. A slightly modified version of the Heller frozen Gaussian approximation lsqb;E. J. Heller, J. Chem. Phys.75, 2923 (1981)rsqb; is proposed and used to obtain the intensities of the vibrational bands. We conclude that it is important to use the open shell molecular orbital basis and the SDCI plus Pople correction, or even better, the CEPA. Both methods give good results for the vertical transition energy and excited state geometry. The error in the vibrational frequencies is in the order of 10percnt;, but theNNhyphen;stretch mode is best described by the CEPA method.