Highly excited vibrational levels of lsquo;lsquo;floppyrsquo;rsquo; triatomic molecules: A discrete variable representationmdash;Distributed Gaussian basis approach

摘要

A novel, efficient, and accurate quantum method for the calculation of highly excited vibrational levels of triatomic molecules is presented. The method is particularly well suited for applications to lsquo;lsquo;floppyrsquo;rsquo; molecules, having large amplitude motion, on potential surfaces which may have more than one local minimum. The discrete variable representation (DVR) for the angular, bend coordinate is combined with the distributed (real) Gaussian basis (DGB) for the expansion of other, radial coordinates. The DGB is tailored to the potential, covering only those regions whereV(r)EMAX. The DVR permits a contraction of the primitive Gaussian basis to a small eigenfunction basis at each of the discretized values of the angular coordinate. It is shown for the floppy twohyphen;mode LiCN/LiNC system (fixed CN distance) thatNlowest vibrational levels (N=131) can be converged to within 1 cmminus;1(the lowest 117 to 0.1 cmminus;1) using only 3Nbasis functions. This appears to reduce the computational effort by a factor of 10ndash;40 over standard methods. Moreover, only a very low order Gaussndash;Hermite quadrature, 3ndash;5 points, is needed to evaluate each potential matrix element.