Entwicklung von Multireferenz-Schwingungskorrelationsverfahren
Zusammenfassung der Projektergebnisse
A series of variational and perturbational multireference methods has been implemented into the M OLPRO package of ab initio programs. These VMRCI and VMRPT2 programs rely on state-specific or ground-state based VMCSCF or VCASSCF wave functions and can deal with grid-based or analytical representations (polynomials, B-splines and distributed Gaussians) of multi-dimensional potential energy surfaces. The VMRCI programs work in a configuration selective manner based on a selection criterion originating from Shavitt’s Ak method. All parts of the program are parallelized and make use of the full Watson Hamiltonian, i.e. vibrational angular momentum terms are included. Those programs, which neglect direct couplings between the configurations included in the active space of the underlying VMCSCF calculation and the secondary space of the correlation calculation, i.e. relaxation effects, lead to consistently poorer results and cannot be recommended for highly accurate calculations. The most powerful multiconfigurational approach was found to be given by the following scheme: 1. Do simple VSCF calculations for all states of interest. 2. Determine mode-specific active spaces for all individual states by just one small VCI calculation. This identifies resonating states, which require a multireference treatment. 3. Perform state-averaged VCASSCF calculations based on small active space. In case that the active space contains just one configuration, this calculation corresponds simply to a state-specific VSCF calculation. 4. Perform configuration selective VMRCI calculations based on the VCASSCF modals. In case of just one reference configuration this program yields the same answer as a singlereference VCI calculation. For most applications we found VMRCI calculations including single and double excitations to be sufficient. However, once the active space is very small, triple excitations should be included. benchmark calculations have been provided for molecules with strongly resonating vibrational states. A VMRPT2 programs leads to significantly faster calculations, but also to less accurate results. Infrared intensities are available for all newly implemented programs.
Projektbezogene Publikationen (Auswahl)
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Selected Aspects Concerning the Efficient Calculation of Vibrational Spectra beyond the Harmonic Approximation. Croat. Chem. Acta 2012, 85, 379
D. Oschetzki, M. Neff, P. Meier, F. Pfeiffer and G. Rauhut
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Multi-reference vibration correlation methods. J. Chem. Phys. 2014, 140, 064110
F. Pfeiffer and G. Rauhut
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Towards an automated and efficient calculation of resonating vibrational states based on state-averaged multiconfigurational approaches. J. Chem. Phys. 2015, 143, 244111
P. Meier, D. Oschetzki, F. Pfeiffer and G. Rauhut
