In microporous materials, poromechanical drained (open pore) and undrained (close pore) conditions play a role in the thermo-mechanical properties, which can also be affected by specific ion effects. Heat capacity is a key property to understanding the thermo-hydro-mechanical behavior of clays. However, there is a lack of bottom-up approaches aiming at capturing the influence of confined water and ion effects on the heat capacity of clays. We perform molecular dynamics simulations to obtain the heat capacity of Na- and Ca-montmorillonite using classical and semi-classical approaches. The heat of adsorption is computed from Grand Canonical Monte Carlo (GCMC) simulations at ambient conditions. We provide the theoretical framework to compute the drained heat capacity using the undrained heat capacity and the heat of adsorption. The semi-classical approach is required to capture the heat capacity of the solid mineral but cannot properly capture that of the inter-layer electrolyte. A hybrid approach is proposed, which combines the semi-classical approach for the mineral and the classical approach for the electrolyte. The undrained and drained heat capacities of clays evolve linearly with the water content, which leads to the conclusion that the effects of the mixing of hydration states are not significant. Undrained heat capacity is almost independent of the counterion nature. In contrast, the overall trends of drained heat capacity plotted against the water content exhibit larger values for Na-Mmt than for Ca-Mmt. Our results are in agreement with experimental data for a variety of smectites.