Vibrational spectroscopy is one of the most useful probes into the properties of molecules. <i>Ab initio</i> anharmonic calculations are currently able to provide accurate results for a number of small molecules. For large systems, computational costs become prohibitive due to the requirement of significant molecular information beyond the local minimum-energy structure in most common anharmonic methods, such as VSCF. The efficiency and accuracy of these methods can strongly depend on the choice of coordinates, and they often require a large degree of mode coupling when handling large anharmonic effects in normal-mode coordinates. In this work, we demonstrate that the use of automatically generated local-mode coordinates enables us to systematically reduce the degree of mode coupling. This approach dramatically reduces the computational cost for large molecules, and linear scaling is predicted for very large systems. Because any geometry in the scans for the n-mode representation in local coordinates only differs from the equilibrium by a few atoms, a new algorithm is proposed to accelerate potential generation at the <i>ab initio</i> level, in which two-electron integrals of equilibrium structure are calculated once and only the differing integrals involving moved atoms need to be computed for all other geometries. This approach has been applied to interpret experimental data of several biomolecules, including the protonated sarcosine-glycine dipeptide, protonated cytosine dimer, and protonated tryptophan.

Anharmonic vibrational spectroscopy calculations of large molecules

251st National Meeting (2016)

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