Evaluation of Uncertainty of Ideal-Gas Entropy and Heat Capacity Calculations by Density Functional Theory (DFT) for Molecules Containing Symmetrical Internal Rotors

The
uncertainty of thermophysical data is indispensable information
when reporting both experimental and calculated values. In this paper,
we present an evaluation of the uncertainty of the ideal-gas entropy
and heat capacity calculations by density functional theory (DFT)
for molecules containing symmetrical internal rotors. The rigid-rotor
harmonic oscillator (RRHO) and one-dimensional hindered rotor (1-DHR)
models are compared as well as the effect of the scale factors employed.
The calculations of the standard ideal-gas entropy (Sg0) are performed for a selected set of 33 molecules for
which reliable reference data were found in the literature. The RRHO
model provides Sg0 with the absolute average
percentage deviations (σr) about 2 % from the reference
data. Scaling the frequencies does not lead to any improvement when
using the RRHO model. A significant improvement is achieved when the
1-DHR model and scale factors for low and high frequencies are applied
simultaneously (σr less than 0.3 %). The ideal-gas
heat capacity (Cpg0) calculations were tested on
a set of 72 molecules. The RRHO model yields Cpg0 values with σr up to 3 % at 300 K and 1 % at 1000
K while using the 1-DHR model coupled with a pair of scale factors
lowers σr to less than 1.5 % and 0.5 % at 300 K and
1000 K, respectively.