title: Automatic determination of important mode-mode correlations in many-mode vibrational wave functions.
abstract: We introduce new automatic procedures for parameterizing vibrational coupled cluster (VCC) and vibrational configuration interaction wave functions. Importance measures for individual mode combinations in the wave function are derived based on upper bounds to Hamiltonian matrix elements and/or the size of perturbative corrections derived in the framework of VCC. With a threshold, this enables an automatic, system-adapted way of choosing which mode-mode correlations are explicitly parameterized in the many-mode wave function. The effect of different importance measures and thresholds is investigated for zero-point energies and infrared spectra for formaldehyde and furan. Furthermore, the direct link between important mode-mode correlations and coordinates is illustrated employing water clusters as examples: Using optimized coordinates, a larger number of mode combinations can be neglected in the correlated many-mode vibrational wave function than with normal coordinates for the same accuracy. Moreover, the fraction of important mode-mode correlations compared to the total number of correlations decreases with system size. This underlines the potential gain in efficiency when using optimized coordinates in combination with a flexible scheme for choosing the mode-mode correlations included in the parameterization of the correlated many-mode vibrational wave function. All in all, it is found that the introduced schemes for parameterizing correlated many-mode vibrational wave functions lead to at least as systematic and accurate calculations as those using more standard and straightforward excitation level definitions. This new way of defining approximate calculations offers potential for future calculations on larger systems.
authors: Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark.
authors: Carolin
authors: C
authors: König
authors: Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark.
authors: Ove
authors: O
authors: Christiansen
mesh_terms: D001331:Automation; D005557:Formaldehyde / Q000737:chemistry; D005663:Furans / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D013055:Spectrophotometry, Infrared; D014732:Vibration; D014867:Water / Q000737:chemistry
chemical_list: D005663:Furans; D014867:Water; D005557:Formaldehyde; C039281:furan