Pulay stress

Pulay stress is unphysical stress resulting from unconverged calculations with respect to basis set. It distorts the cell structure, decreasing it from the equilibrium volume and creating difficulties in volume relaxation. The resultant energy vs. volume curves, cf. Figure 1 (top), are jagged and special care must be taken to obtain reasonable structures, cf. volume relaxation PAGE-LINK. In this article, the computational origin of this is discussed. It is important to note that problems due to the Pulay stress can often be neglected if only volume-conserving relaxations are performed. This is because the Pulay stress is, usually, nearly uniform and only changes the diagonal elements of the stress tensor by a constant amount.

Figure 1. Total energy (left y-axis) and absolute pressure (right y-axis) vs. lattice parameter. Equilibrium lattice parameters for energy and pressure are shown. These coincide when Pulay stress is eliminated. ENCUT = 250 eV (top - unconverged) and 540 eV (bottom - converged). Diamond in a primitive cell - 2x2x2 k-point mesh.

Introduction

The energy for a periodic system, e.g. band structures, is calculated using a finite number of plane waves and a finite number of k-points. A fixed number of plane waves or plane wave energy cutoff may be used to set a constant basis. In VASP, a constant energy cutoff is used, cf. ENCUT. The number of plane waves $N_{PW}$ is related to the energy cutoff $E_{cutoff}$ and the size of the cell $\Omega$:

$N_{PW} \propto\ \Omega\ E_{cutoff}^{3/2}$

$N_{PW}$ is constant in a relaxation calculation, which means that $E_{cutoff}$ must change to compensate for changes in $\Omega$. All the initial G-vectors within a sphere are included in the basis. However, when comparing cells of different sizes, i.e. during a relaxation, the cell shape is relaxed, so the direct and reciprocal lattice vectors change. The number of reciprocal G-vectors in the basis is kept fixed but the length of the G-vectors changes, indirectly changing the energy cutoff. In other words, the shape of the cutoff region changes from a sphere to an ellipsoid. This can be solved by using an infinite number of k-points and plane waves. In practice, a large enough plane wave energy cutoff and number of k-points leads to converged energies. All energy changes are strictly consistent with the stress tensor; however, when the basis set is too small, i.e. prematurely truncated, this results in discontinuities in the total energy between cells of varying volumes. These discontinuities between energy and volume create stress that decreases the equilibrium volume (cf. Fig. 1 (top)), due to the diagonal components of the stress tensor being incorrect. This is called the Pulay stress.

The pressure of the cell, being proportional to the trace of the stress tensor, can be used to visualize this. When the cell volume is below the equilibrium volume, the pressure is positive; contrastingly, it is negative when above the equilibrium volume, so at equilibrium, this is zero. Plotting the magnitude of the pressure vs. volume curve and the total energy allows comparison between these two minima. In Figure 1, when focusing on the energy-volume curve, it is clear that the equilibrium volume is roughly similar between the top and bottom curves, even though the top curve is jagged and the bottom is not. This is why fitting an equation of state to energy-volume curves with small bases still gives reasonable volumes. A very different picture is seen for the absolute pressure-volume curves, where the pressure/ energy minima coincide for a converged basis, while the pressure equilibrium is much lower than the energy equilibrium. This is the effect of the Pulay stress.