若只进行体积守恒的弛豫，通常可以忽略 Pulay stress，因为 pulay 应力几乎是均匀的。
这一步不得不增大 ENCUT，（但整个计算 ENCUT 轻易是不变的），是这一步例外，一旦计算完晶格常数，可以在这个基础上统一使用其他的 ENCUT。
Volume vs. energy, volume relaxations, Pulay Stress
If you are doing energy-volume calculations or cell shape and volume relaxations you must understand the Pulay stress, and related problems.
The Pulay stress arises from the fact that the plane wave basis set is not complete with respect to changes of the volume. Thus, unless absolute convergence with respect to the basis set has been achieved - the diagonal components of the stress tensor are incorrect. This error is often called "Pulay stress". The error is almost isotropic (i.e. the same for each diagonal component), and for a finite basis set it tends to decrease volume compared to fully converged calculations (or calculations with a constant energy cutoff).
The Pulay stress and related problems affect the behavior of VASP and any plane wave code in several ways: First it evidently affects the stress tensor calculated by VASP, i.e. the diagonal components of the stress tensor are incorrect, unless the energy cutoff is very large (ENMAX=1.3 * default is usually a safe setting to obtain a reliable stress tensor).