Plane-Wave DFT

Plane-wave DFT in Rowan is run through Quantum ESPRESSO and supports periodic systems such as bulk crystals, surfaces, and defects.

Plane-wave DFT calculations can be controlled using the following settings:

• Pseudopotentials: element-specific effective potentials replacing the core electrons. (Rowan selects from the SSSP library.)
• (Optional) Energy cutoffs: the kinetic-energy thresholds defining the plane-wave basis.
• (Optional) K-points: the reciprocal-space grid at which the electronic structure is sampled.
• (Optional) Smearing: broadening of the Fermi-level occupation, used for metals.

Pseudopotentials

A pseudopotential replaces an element's chemically inert core electrons and the nuclear potential with a single smooth effective potential, reproducing the correct behavior outside the core region. Valence electrons, which determine bonding, conductivity, and reactivity, are treated explicitly. Three families are in common use: norm-conserving, ultrasoft, and projector-augmented wave (PAW).

Rowan uses the Standard Solid-State Pseudopotentials (SSSP) library, a curated per-element mix of norm-conserving, ultrasoft, and PAW pseudopotentials. Four variants are available:

• SSSP_PBE_efficiency: PBE pseudopotentials, optimized for throughput.
• SSSP_PBE_precision: PBE pseudopotentials, optimized for accuracy.
• SSSP_PBEsol_efficiency: PBEsol pseudopotentials, optimized for throughput.
• SSSP_PBEsol_precision: PBEsol pseudopotentials, optimized for accuracy.

The pseudopotential variant should match your functional (PBEsol with PBEsol, PBE with PBE and all other non-PBEsol functionals).

Energy Cutoffs

Two energy cutoffs define the plane-wave basis: one for the wavefunctions (plane-wave cutoff) and one for the charge density (charge-density cutoff). In each case, all plane waves with kinetic energy below the cutoff are included. Each is element-specific, and unless otherwise specified, Rowan sets them automatically to the maximum of the SSSP-recommended values across all elements present.

For high-accuracy calculations, it is standard practice to raise the cutoffs by approximately 20% above the recommended values.

K-Points

The electronic structure of a periodic solid varies across reciprocal space and must be sampled on a discrete grid. K-points are the sample locations on that grid.

Rowan generates a Monkhorst–Pack k-point grid by spacing k-points at approximately 0.3 Å⁻¹ in reciprocal space, with the grid dimensions derived from the input cell. The default grid is independent of material type.

• Insulators and semiconductors: the default density is generally sufficient.
• Metals: denser sampling is typically required, together with smearing (see below).

Smearing

In a metal, states at the Fermi level abruptly switch from fully occupied to fully empty as you move through reciprocal space, which makes k-point sampling unstable. Smearing softens this boundary into a smooth transition, allowing the integration to converge.

Smearing is not applied automatically and must be set by the user. Rowan exposes four standard schemes:

• Marzari–Vanderbilt (cold smearing): recommended for most metals.
• Methfessel–Paxton: alternative for metals; good total-energy convergence.
• Fermi–Dirac: physical thermal occupation; mainly for finite-temperature studies.
• Gaussian: simplest broadening; less accurate for total energies.

A starting sigma of ≈0.01 Hartree is reasonable for most metals. For insulators and semiconductors, typically leave smearing off.