The impact of highly correlated potential energy surfaces on the anharmonically corrected IR spectrum of acetonitrile.
This paper discusses the quality and feasibility of highly correlated ab initio techniques in a vibrational self-consistent field (VSCF) approach using acetonitrile as a model system. The topical renormalized coupled-cluster technique exploiting the similarity-transformed Hamiltonian's left eigenstates (i.e. CR-CC(2,3)) is investigated alongside the well-known Hartree-Fock (HF), Møller-Plesset second-order perturbation theory (MP2) and coupled cluster (CCSD(T)) methods. The inclusion of mode triple interactions is discussed and it is found that the use of an effective core potential (ECP) serves as a viable compromise during the highly demanding task of computing such contributions, thus enabling a grid-based evaluation of three mode interaction terms with coupled cluster techniques also for larger molecules. In this context, a previously proposed reduced coupling scheme [1] is investigated, confirming the applicability of this technique to a system exhibiting a rather complex electronic structure. A combination of Ahlrichs' triple-ζ valence polarized (TZVP) basis set with Dunning's set of core-valence correlation functions is found to deliver results in good agreement with experiment while being computationally very feasible. Since CH3CN exhibits four degenerate vibrational degrees of freedom, it serves as an ideal model system for critically assessing the qualities of the degenerate second-order perturbation theory corrected (DPT2) VSCF technique. Besides fundamental vibrations, a thorough investigation of overtone transitions and combination bands is conducted by means of comparing the results to both available and newly recorded experimental data.
Institute of Analytical Chemistry and Radiochemistry, Center for Chemistry and Biomedicine, Leopold-Franzens University, Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria.
Oliver M D
OM
Lutz
Theoretical Chemistry Division, Institute of General, Inorganic and Theoretical Chemistry, Center for Chemistry and Biomedicine, Leopold-Franzens University, Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria.
Bernd M
BM
Rode
Institute of Analytical Chemistry and Radiochemistry, Center for Chemistry and Biomedicine, Leopold-Franzens University, Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria.
Günther K
GK
Bonn
Institute of Analytical Chemistry and Radiochemistry, Center for Chemistry and Biomedicine, Leopold-Franzens University, Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria. Electronic address: christian.w.huck@uibk.ac.at.
Christian W
CW
Huck
D000097:Acetonitriles / Q000737:chemistry*; D008958:Models, Molecular; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared / Q000379:methods
D000097:Acetonitriles; C032159:acetonitrile
Anharmonic; CR-CC(2,3); Computational spectroscopy; Overtones; VSCF; Vibrational