What Is More Important When Calculating the Thermodynamic Properties of Organic Crystals, Density Functional, Supercell, or Energy Second-Order Derivative Method Choice?

Calculation of second-order derivatives of energy using the DFT method is a valuable approach for the estimation of both the thermodynamical and mechanical properties of organic crystals from the first principles. This type of calculation requires specification of several computational parameters, including the functional, supercell, and method of phonon calculations. Nevertheless, the importance of these parameters is presented in the literature very modestly. In this work, we demonstrate the influence of these computational parameters on the accuracy of calculated second-order derivatives using the practical example of pyrazinamide polymorphs, including the plastically bending α form and the β, γ, and brittle δ form. The effects of the settings used on the resulting enthalpies of the polymorphic modifications of pyrazinamide are compared: supercell setting (primitive cell vs. appropriate supercell) has a much stronger impact than functional (PBE-D3BJ vs. Hamada rev-vdW-DF2) which in turn affects results significantly more than the method for second-order derivative computation (FD vs. DFPT approach). Finally, we propose some suggestions for choosing the right settings for calculating second-order derivatives for molecular crystals.