Toward Accurate Theoretical Vibrational Spectra: A Case Study for Maleimide

We
employ and combine a number of recent developments in vibrational
structure methods to push their current size limitations toward molecules
with tens of modes and showcase their availability for the maleimide
molecule. In particular, we assess the use of different rectilinear
vibrational coordinates, namely, normal coordinates, hybrid optimized
and localized coordinates, and flexible adaptation of local coordinates
of nuclei coordinates. These different coordinate parameterizations
are employed in conjunction with the adaptive density-guided approach
to generate potential energy surfaces (PESs). A screening procedure
is furthermore introduced, which provides estimates of the importance
of individual terms in the PES, resulting in significant reductions
in the computational cost of the PES construction. We find that all
three sets of coordinates provide approximately the same level of
accuracy in vibrational structure calculations and report fundamental
excitation energies with a mean absolute deviation of less than 12
cm–1 when compared to experimental data. We expect
that similar accuracy in vibrational structure calculations can be
achieved for molecules of larger size using the proposed procedures.