4.2. Standards and Reference Energies
Contents
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4.2.1. The MedeA Standard 500
MedeA has introduced a standard for the Precision of VASP calculations, automatically applying the directive PREC = Accurate and a plane wave cutoff of 500 eV (900 eV for hard potentials with ENMAX > 450 eV). The MedeA VASP GUI option Standard 500 is recommended in cases where an overall precise plane cutoff and setting is required for a single system or for a range of systems with varying constituent elements. Using a standard cutoff of 500eV within a project has the advantage of facilitating the comparison of data from compounds containing different elements like e.g. in the calculation of heat of formations or defect energies.
VASP parameters covered by :highlightgray:`Precision` `Standard 500`:
- Precision accurate
- Planewave cutoff 500eV (900 eV for hard potentials with ENMAX > 450 eV)
VASP parameters recommended to be set in addition, to comply with `Standard 500`:
- Reciprocal space projection
- Convergence criterion for SCF cycle: 10-7 eV
- Convergence criterion for geometry optimization: 0.001 eV/\({\mathring{\mathrm{A}}}\)
- Conjugate gradient geometry optimization
For crystalline structures:
- k-spacing of 0.2 \({\mathring{\mathrm{A}}}\)-1, Shift origin to Gamma
- For non-local exchange based functionals (e.g. hybrid functionals): k-spacing of 0.5 \({\mathring{\mathrm{A}}}\)-1, shift origin to Gamma, X, Y, and Z axis k-mesh factors all set to 2
- Tetrahedron method including Bl\({\ddot{\mathrm o}}\)chl corrections
For molecular structures:
- Box ensuring a minimum of about 8 \({\mathring{\mathrm{A}}}\) empty space between the molecule and its symmetry copies in all directions
- \({\Gamma}\)-point only
- Fermi smearing with 0 eV smearing width
4.2.2. Reference Energies for the Calculation of the Heat of Formation
The computation of the electronic contribution to heats of formation for compounds requires reference energies for all constituent elements in their standard state. Below is a list of model structures for all elements in their standard state, which is applied for calculating the Energy of formation as a property from the MedeA VASP 6 GUI. For a few cases the standard state structure is replaced by a different structure together with a correction energy. For the structure optimization calculations for the reference systems, in general a non-magnetic Hamiltonian is used, however, spin-polarization is required for O2, Cr(antiferromagnetic), \({\gamma}\)-Mn(antiferromagnetic), Fe, Co, Ni, and lanthanides heavier than Ce for potentials including f electrons as valence states. For solids, all cell parameters and internal degrees of freedom need to be relaxed. For atoms and molecules a fixed box of 10 \({\mathring{\mathrm{A}}}\) in each dimension is used (for S8 molecules a cubic box of 15 \({\mathring{\mathrm{A}}}\)), for molecules the atomic positions need to be relaxed. For computing energies of formation, the application of Standard 500 settings as outlined above are recommended to obtain suitable accuracy.
| Elements | Structure | Correction, techn. details |
|---|---|---|
| H | H2 molecule | |
| He | He atom | |
| Li | bcc | |
| Be | \({\alpha}\)-Be, hcp | |
| B | \({\alpha}\)-B, R-3m | |
| C | diamond, Fd-3m | -1.897 kJ/mol, because graphite is low temperature phase |
| N | N2 molecule | |
| O | O2 molecule | spin-polarized, because molecule exists in triplet state |
| F | F2 molecule | |
| N | Ne atom | |
| Na | bcc | |
| Mg | hcp | |
| Al | fcc | |
| Si | Fd-3m | |
| P | Cmca, black phosphorus | |
| S | S8 molecule | -13.04875 kJ/mol, condensation |
| Cl | Cl2 molecule | |
| Ar | Ar atom | |
| K | bcc | |
| Ca | \({\alpha}\)-Ca, fcc | |
| Sc | \({\alpha}\)-Sc, hcp | |
| Ti | \({\alpha}\)-Ti, hcp | |
| V | bcc | |
| Cr | \({\alpha}\)-Cr, bcc, antiferromagnetic | spin-polarized |
| Mn | \({\gamma}\)-Mn P4/mmm, antiferromagnetic | -4.348 kJ/mol = \({\alpha}\)-Mn, spin-polarized |
| Fe | \({\alpha}\)-Fe, bcc | spin-polarized |
| Co | \({\beta}\)-Co, hcp | spin-polarized |
| Ni | fcc | spin-polarized |
| Cu | fcc | |
| Zn | hcp | |
| Ga | \({\alpha}\)-Ga, Cmca | |
| Ge | Fd-3m | |
| As | R-3m | grey metallic |
| Se | P3121 | grey metallic |
| Br | Br2 molecule | -22.85 kJ/mol, condensation |
| Kr | Kr atom | |
| Rb | bcc | |
| Sr | \({\alpha}\)-Sr, fcc | |
| Y | hcp | |
| Zr | \({\alpha}\)-Zr, hcp | |
| Nb | bcc | |
| Mo | bcc | |
| Tc | hcp | |
| Ru | hcp | |
| Rh | fcc | |
| Pd | fcc | |
| Ag | fcc | |
| Cd | hcp | |
| In | I4/mmm | |
| Sn | \({\alpha}\)-Sn, Fd-3m | transition temperature of 286 K to \({\beta}\)-Sn, I41/amd |
| Sb | R-3m | |
| Te | P3121 | |
| I | Cmca | |
| Xe | Xe atom | |
| Cs | bcc | |
| Ba | bcc | |
| La | \({\alpha}\)-La, dhcp | |
| Ce | \({\gamma}\)-Ce, fcc | |
| Pr | dhcp | |
| Nd | dhcp | |
| Pm | dhcp | |
| Sm | R-3m | |
| Eu | bcc | |
| Gd | hcp | |
| Tb | hcp | |
| Dy | hcp | |
| Ho | hcp | |
| Er | hcp | |
| Tm | hcp | |
| Yb | fcc | |
| Lu | hcp | |
| Hf | hcp | |
| Ta | \({\alpha}\)-Ta, bcc | |
| W | \({\alpha}\)-W, bcc | |
| Re | hcp | |
| Os | hcp | |
| Ir | fcc | |
| Pt | fcc | |
| Au | fcc | |
| Hg | R-3m | 2.3 kJ/mol melting (JANAF) |
| Tl | \({\alpha}\)-Tl, hcp | |
| Pb | fcc | |
| Bi | R-3m | |
| Po | Pm-3m | |
| At | no structure | |
| Rn | Rn atom | |
| Fr | no structure | |
| Ra | bcc | |
| Ac | fcc | |
| Th | \({\alpha}\)-Th, fcc | |
| Pa | I4/mmm | |
| U | \({\alpha}\)-U, Cmcm | |
| Np | Pnma | |
| Pu | \({\alpha}\)-Pu, P121/m1 | |
| Am | dhcp | |
| Cm | dhcp |
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