title: Fast vibrational calculation of anharmonic OH-stretch frequencies for two low-energy noradrenaline conformers.
abstract: We introduce a new reduced-coupling technique to accelerate direct calculations of a selected number of vibrational frequencies in large molecular systems. Our method combines the advantages of the single-to-all correlation-corrected vibrational self-consistent field (STA-CC-VSCF) approach [D. M. Benoit, J. Chem. Phys. 125, 244110 (2006)] with those of the fast-CC-VSCF technique [D. M. Benoit, J. Chem. Phys. 120, 562 (2004)] and allows the ab initio calculation of only the relevant parts of the required potential energy surface (PES). We demonstrate, using a set of five aliphatic alcohol molecules, that the new fast-STA-CC-VSCF method is accurate and leads to very substantial time gains for the computations of the PES. We then use the fast-STA-CC-VSCF method to accelerate the computation of the OH-stretch and NH-stretch frequencies of the two lowest-energy conformers of noradrenaline, namely, AG1a and GG1a. Our new approach enables us to run the calculation 89 times faster than the standard CC-VSCF technique and makes it possible to use a high-level MP2/TZP description of the PES. We demonstrate that the influence of the strong mode-mode couplings is crucial for a realistic description of the particular OH-stretch vibrational signature of each conformer. Finally, of the two possible low-energy conformers, we identify AG1a as the one most likely to have been observed in the experiments of Snoek et al. [Mol. Phys. 101, 1239 (2003)].
authors.affiliation: Nachwuchsgruppe Theorie-SFB 569, Albert-Einstein-Allee 11, University of Ulm, D-89081 Ulm, Germany. david.benoit@uni-ulm.de
authors.forename: David M
authors.initials: DM
authors.lastname: Benoit
mesh_terms: D006878:Hydroxides / Q000737:chemistry*; D007536:Isomerism; D008968:Molecular Conformation; D009638:Norepinephrine / Q000737:chemistry*; D012680:Sensitivity and Specificity; D013816:Thermodynamics; D013997:Time Factors; D014732:Vibration
chemical_list: D006878:Hydroxides; C031356:hydroxide ion; D009638:Norepinephrine