Isoreticular MOF-74 structures are an ideal case for understanding the role of metal on the heat capacity as it has been made with a wide range of metals, including Mg, Mn, Fe, Ni, Co, and Zn. These materials have been studied extensively for carbon capture. Figure shows the computed heat capacity of these materials at room temperature, which can indeed vary considerably with the change of metal. For example, the Mg-MOF-74 shows the lowest molar heat capacity amongst these six metals. To explain this observation, we inspect the lattice vibrational frequencies of these systems. As the only difference between these structures are their metal centers, we focus on the lattice vibrational frequencies of the metal centers. Figure shows the histograms of the projected lattice vibrational frequencies of metal centers for Mg and Zn MOF-74, where we see that Zn has a shift in the vibrational frequencies to lower frequencies compared to Mg. Theoretically, different frequencies contribute to the heat capacity differently: At low and intermediate temperatures, the low-frequency modes are dominant in contributing to the heat capacity, and only at high temperatures the high-frequency modes also participate in the heat capacity (see Figure for a quantitative picture). Substituting Mg with Zn shifts the vibrational frequencies to lower frequencies, and hence, leading to a higher heat capacity in Zn-MOF-74. One can explain the effect of changing the mass of the metal node if we envision this metal to be connected with harmonic springs to its neighbors. The vibrational frequency of this metal is proportional to k /m eff , where k is the spring stiffness and m eff is the effective mass. If change of the mass would be the only contribution, one would expect monotonically increasing heat capacity with increasing the molecular mass of the metal. Figure shows that differences in mass only partly explains the data; when the masses are similar, the details of the metal-linker interactions are also important. It is important to note that in practice, the heat capacity is reported per gram of material. As the weight of the material increases more than the heat capacity, we typically see a decrease of the heat capacity if plotted per gram of material instead of per atom.