The heatmap in Figure illustrates the deviations in reaction enthalpies (in kcal/mol) calculated using various DFT methods with the 6-31G(d) basis set, relative to experimental values. The color gradient reflects the magnitude of deviation, with yellow indicating smaller deviations and purple signifying larger errors. A clear trend emerges as the number of carbon atoms increases. For all DFT methods, the deviation becomes progressively larger with increasing chain length. Smaller molecules like methane (n=1) show relatively minor deviations, with values in the range of 30-50 kcal/mol. In contrast, larger alkanes, such as decane (n=10), exhibit significantly higher deviations, often exceeding 200 kcal/mol. This trend highlights the scaling challenges associated with accurately modeling combustion enthalpies for long-chain hydrocarbons due to increased complexity. Different DFT methods exhibit varying performance. PBEPBE and TPSSh demonstrate moderate deviations but fail to converge for n=6, indicating computational challenges or methodological limitations. In contrast, B3LYP, B2PLYP, and B2PLYPD3 perform significantly better, particularly for smaller alkanes, with lower deviations across the board. Interestingly, the simplest Rung-1, LSDA functional yields the smallest deviations overall, indicating that added complexity in higher-rung methods does not always translate to improved accuracy. An important feature of the heatmap with split-valence basis set is the "Unconverged" entries for PBEPBE and TPSSh at n=6. The reason is explained at the later part in this section with the heading, 3.5 The unconvergence of nhexane. Figure illustrates the percent deviation in reaction enthalpies calculated using various DFT methods with the split-valence basis set. Among the methods, LSDA consistently shows the lowest percent deviation, while TPSSh exhibits the highest across all alkane sizes. Interestingly, as the size of the alkane increases, the percent deviation decreases, which is the opposite trend observed for absolute deviation. This occurs because the experimental reaction enthalpy increases with the number of carbon atoms, causing the relative deviation to appear smaller for larger molecules.