Mode-mode coupling maps constructed for methanol, using normal modes from a Hessian. The PES was evaluated with (a) , (b) PM3, and (c) the Harris functional. The values of the pair-coupling strength, , are color coded from white to black . The selected couplings for a fast-(DC)VMP2 calculation are indicated by white dots.
Histograms and box plots (see text for details) for the normalized and symmetrized pair-coupling potential measure, , for methanol from a Hessian. The values of are obtained from a PES calculation with (a) , (b) PM3 [“”], and (c) the Harris functional [“”]. The fit to a normal distribution is shown on the plot by a dashed line.
Full DCVMP2 and fast-DCVMP2 vibrational frequencies for methanol in , at the level of theory. The coupling-strength parameter is calculated from a , a PM3, and a Harris functional PES. The threshold for the fast-DCVMP2 calculation is selected from box plots. Uncoupled (diagonal) frequencies are displayed in square brackets. MAD (MAXAD) is the mean (maximum) absolute deviation, and RMSD the root-mean-square deviation, from the full DCVMP2 frequencies.
Deviations for the calculated harmonic, diagonal, fast-DCVMP2, and full DCVMP2 frequencies for mercaptomethane and pyridine (MP2/TZP) from experiment (Refs. 66 and 67); in . MAD (MAXAD) is the mean (maximum) absolute deviation, and RMSD the root-mean-square deviation.
Calculated fast-VMP2/fast-DCVMP2, full VMP2/DCVMP2, and full VCI frequencies [ for methanol and MP2/TZP for mercaptomethane and pyridine]; in . Differences are shown in parentheses.
Harmonic, fast-DCVMP2, and experimental frequencies for 4-mercaptopyridine (MP2/TZP), and normal-mode assignments; in . Uncoupled (diagonal) frequencies among the fast-DCVMP2 frequencies are shown in square brackets. The fast-DCVMP2 frequency for torsional mode is negative, due to an overcorrection in the perturbative treatment (see text).
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