Articles you may be interested inA multidimensional pseudospectral method for optimal control of quantum ensembles A classical determination of vibrationally adiabatic barriers and wells of a collinear potential energy surfaceWe describe a new algorithm for computing eigenvalues, spectral intensities, and selected eigenvectors of multidimensional vibrational potential surfaces. The method involves a synthesis of pseudospectral and sequential adiabatic reduction methods and merges the storage and computational advantages of the former with the improved basis set generated by the latter. The recursive residue generation method, which utilizes a Lanczos-based diagonalization procedure, is employed to calculate the observables. As a test case, we apply the method to computation of the infrared and stimulated emission pumping spectra for the HCN molecule and demonstrate a very large (one to three orders of magnitude) reduction in computational effort (for comparable accuracy) as compared to discrete variable representation (DVR)/adiabatic reduction or standard collocation approaches. We expect that this advantage will be increased considerably for larger (e.g., tetra-atomic) systems and will permit accurate basis set calculations on such systems to be carried out in a straightforward fashion.324

We present a potential energy surface for the HCN/HNC system which is a fit to extensive, high quality ab initio, coupled-cluster calculations. The new surface is an improved version of one that was reported previously by us [J. A. Bentley, J. M. Bowman, B. Gazdy, T. J. Lee, and C. E. Dateo, Chem. Phys. Lett. 198, 563 (1992)]. Exact vibrational calculations of energies and wave functions of HCN, HNC, and delocalized states are done with the new potential using a new method, which combines a truncation/recoupling method in a finite basis representation procedure with a moveable basis to describe the significant bend–CH stretch correlation. All HCN and HNC states with energies below the energy of the first delocalized state are reported and characterized. All delocalized states up to 18 347 cm−1 above the HCN zero-point energy and higher energy localized HCN states are also reported and characterized. Vibrational transition energies are compared with all available experimental data on HCN and HNC, including high CH-overtone states up to 23 063 cm−1. We also report a simulation of the ÖX̃ stimulated emission pumping (SEP) spectrum, and compare the results to experiment. The simulation is performed within the Franck–Condon approximation, and makes use of 400 even-bend wave functions for the ground electronic state, and a realistic vibrational wave function for the first excited bend state in the excited à state. The potential for the à state is slightly modified, relative to one implied by a previously reported force field, to improve agreement with the experimental fundamentals for the à state. In addition, the Ã-state wave function is adjusted slightly to improve agreement with the SEP spectrum. We also report Franck–Condon factors for odd bending states of HCN, with one quantum of vibrational angular momentum, in order to compare with the recent assignment by Jonas, Yang, and Wodtke [J. Chem. Phys. 97, 2284 (1992)], based on axis-switching arguments of a number of previously unassigned states in the SEP spectrum.

A methodology for the calculation of high energy vibrational eigenstates of S0 acetylene is described. Acetylene is modeled as a 5D planar molecule. The discrete variable representation (DVR) is employed for radial coordinates with a finite basis representation (FBR) for the angular coordinates. Symmetry adaptation of the primitive basis (dimension 2.7 × 106) coupled with a two level diagonalization/truncation scheme maintains relatively small basis sets (< 2500 functions) in all diagonalizations. Eigenvalues up to nearly 3700 cm−1 above the ground state are reported.

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Articles you may be interested inAb initio calculation of a global potential, vibrational energies, and wave functions for HCN/HNC, and a simulation of the A-X̃ emission spectrum J. Chem. Phys. 99, 308 (1993); 10.1063/1.465809 Highly excited vibration-rotation states of floppy triatomic molecules by a localized representation method: The HCN/HNC molecule J. Chem. Phys. 93, 3039 (1990); 10.1063/1.458838 The extraction of dynamics and the classical mechanical simulation of low resolution regular and chaotic spectra: HCN/HNC J. Chem. Phys. 93, 76 (1990); 10.1063/1.459529 The vibrational spectrum and the isomerization potential of HCN/HNC J. Chem. Phys. 92, 3633 (1990); 10.1063/1.457819 Accurate localized and delocalized vibrational states of HCN/HNCWe report the large scale calculation of accurate vibrational eigenstates of X state HCN/ HNC up to -37 000 cm-t . The discrete variable representation-distributed Gaussian basis (DVR-DGB) approach is used. In the final major computational step, large dense Hamiltonian matrices (> 11 000 rows or columns) must be diagonalized. The use of new software for out-of-core eigenproblems is described in some detail. Representative wave functions of high lying vibrational eigenstates near the isomerization barrier and at energies up to 39 313 cm -I_are presented. Theoretical stimulated emission pumping (SEP) spectra for several initial (A) vibrational states are presented along with calculations of the quantum survival probability for these spectra. The spectra range from being dominated by localized eigenstates to being dominated by delocalized eigenstates, depending on the geometry .of each initial state. In addition, time-dependent expectation values of the coordinates of the evolving wave packet are plotted.

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