Bela Gazdy

Ab initio calculation of a global potential, vibrational energies, and wave functions for HCN/HNC, and a simulation of the (A-tilde)-(X-tilde) emission spectrum

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...

A method to constrain vibrational energy in quasiclassical trajectory calculations

In this paper, we present a general method to constrain the classical energy of a vibrational mode to be greater than a specifled amount. In particular, zero‐point energy constraints can be applied with this method to (zero‐order) vibrational modes of a polyatomic system or complex. A demonstration of the method is made for a model two‐mode Henon–Heiles Hamiltonian.

A TRUNCATION/RECOUPLING METHOD FOR BASIS SET CALCULATIONS OF EIGENVALUES AND EIGENVECTORS

We describe a new method to truncate and recouple basis functions in general variational calculations based on a direct‐product representation of multidimensional wave functions. The method is presented for molecular vibrations; however, the procedure is quite general and can be used in any basis set expansion method. The direct‐product Hamiltonian matrix H is decomposed into a block diagonal matrix H0 plus a remainder H1 . A new subset of basis functions is obtained by diagonalizing H0 . This subset of basis functions is shown to be eigenfunctions of a Hamiltonian in a reduced dimensionality space, ‘‘dressed’’ by the remaining degrees of freedom. These dressed eigenfunctions are then augmented by the component of the original direct‐product basis in which H0 is diagonal. The new basis is recoupled using an energy selection criterion, yielding a substantial reduction in the size of the final full Hamiltonian matrix. The method also suggests a generalization of the vibrational self‐consistent field method,...

A simple method to adjust potential energy surfaces: Application to HCO

A simple method is described to adjust a potential energy surface to improve agreement with experiment. An application is made to the ab initio HCO potential energy surface.(AIP)

An adjusted global potential surface for HCN based on rigorous vibrational calculations

We report extensive trial and error modifications of the Murrell–Carter–Halonen potential surface for HCN to improve agreement with experiments on highly excited stretching and bending states. The vibrational calculations make use of an exact Hamiltonian for nonrotating HCN and use an exact formalism to obtain energies. Two experimental data bases are used to compare against the calculations. One is for highly excited stretch states, but with no bend excitation, and the other is for highly excited bend and CN stretch states, but with no CH stretch excitation. The combined data base consists of 58 vibrational energies for nonrotating HCN. The modifications applied are angular and stretch coordinate scaling and an angular‐dependent potential scaling. In addition, the saddle point position is adjusted to agree with the results of a recent ab initio calculation.

A three‐dimensional L2 simulation of the photodetachment spectra of CIHCI− and IHI−

We report three‐dimensional L2 basis‐set calculations of eigenvalues and eigenfunctions of CIHCI and IHI for zero total angular momentum. Comparisons are made to previous calculations of resonance energies and the bound state in IHI. These eigenfunctions are used in simulations of the photodetachment spectra of ClHCl−→CIHCI+e− and IHI−→IHI+e−. The spectra are convoluted with Gaussian weight functions as was done in very recent simulations of Schatz, based on coupled‐channel scattering calculations, and in the experiments of Neumark and co‐workers.

A global ab initio potential for HCN/HNC, exact vibrational energies, and comparison for experiment

Abstract An ab initio, i.e. from first principles, calculation of vibrational energies of HCN and HNC is reported. The vibrational calculations were done with a new potential derived from a fit to 1124 ab initio electronic energies, which were calculated using the highly accurate CCSD(T) coupled-cluster method in conjunction with a large atomic natural orbital basis set. The properties of this potential are presented, and the vibrational calculations are compared to experiment for 54 vibrational transitions, 39 of which are for zero total angular momentum, J = 0, and 15 of which are for J = 1. The level of agreement with experiment is unprecedented for a triatomic with two non-hydrogen atoms, and demonstrates the capability of the latest computational methods to give reliable predictions on a strongly bound triatomic molecule at very high levels of vibrational excitation.

Theoretical studies of the reactivity and spectroscopy of H+CO=HCO. I. Stabilization and scattering studies of resonances for J=0 on the Harding ab initio surface

We report stabilization and coupled‐channel scattering calculations of isolated resonances for a triatomic molecule HCO using a global, ab initio potential energy surface. The lowest nine resonances are identified for total angular momentum J=0. The two set of calculations agree closely with each other on the resonance locations which are then compared to experimentally determined locations. These resonances show widths varying 10−6 to 100 cm−1. Pictures of resonance wave functions are presented for the proper assignment of quantum numbers of resonance states. A simple technique for locating very narrow resonances in scattering calculations is presented and a worked example shows large savings of computational efforts for scattering calculations. Partial widths and resonance lifetimes are obtained by analysis of the S matrices. Final state distribution probabilities are presented for resonance and direct scattering and the characteristics of this scattering are discussed.

A movable basis method to calculate vibrational energies of molecules

A new basis‐set representation of the vibration/rotation eigenfunctions of triatomic molecules in mass‐scaled Jacobi coordinates is presented. The basis is a nondirect product, consisting of radial basis functions in which the centers and ‘‘shapes’’ are functions of the angular variable. The functional dependence of these parameters is arbitrary, thus giving the method the ability to move the radial basis anywhere in the angular space. This results in a basis with the potential to describe considerable coordinate–coordinate correlation. The advantage of this is noted in the context of a new formulation of self‐consistent field theory, in which a single product function of the above type is variationally optimized. A simple version of the theory, in which only one basis is movable, is applied to two model potentials representing isomerization. The convergence properties are shown to be dramatically better than those using a conventional direct‐product basis, especially for delocalized states, and for the m...

Theoretical stabilization and scattering studies of resonances in the addition reaction H+CO = HCO

We report the first coordinated stabilization and coupled channel scattering calculations of resonances for a realistic, triatomic molecule, HCO, using a global, ab initio potential‐energy surface. The two set of calculations are in excellent agreement with each other for the nine resonance energies reported.