CPD Parameter Guide

This page gives an explanation for the parameters included in CPD entries. The CPD currently contains density functional theory-computed surface adsorption energies. The parameters reflect this application.


Bulk Properties

The properties of the bulk material from which a surface slab or cluster model has been generated for the adsorption energy calculation.

Bulk Formula

The chemical formula of the bulk material.
String
Required

Lattice Constants, Å (x3)

The lattice constants, in angstroms, give the edge dimensions of the unit cell in the bulk crystal structure for the material. Depending on the symmetry of the unit cell (Laue class), 1–3 may be reported.
Numeric

Primary Class

Metadata describing primary characteristics of the bulk material, such as "transition metal".
String

Secondary Class

Metadata describing secondary characteristics of the bulk material, such as "bulk alloy".
String

Space Group

The space group describes the three-dimensional symmetry of the bulk crystal structure.
String

Stretched?

Some calculations introduce a bulk lattice deformation. This field indicates whether the lattice was stretched relative to its relaxed (optimized) lattice parameters.
Boolean
Required

Compressed?

Some calculations introduce a bulk lattice deformation. This field indicates whether the lattice was compressed relative to its relaxed (optimized) lattice parameters.
Boolean
Required

Surface/Particle Properties

The properties of the surface slab or cluster model generated for the adsorption energy calculation.

Facet

Which crystalline facet was used to form the surface slab model? Using Miller index notation.
String

Surface Termination

For many multicomponent materials, after defining a particular facet (cut angle), there are multiple possible surface terminations depending on the precise cut point along the Miller index direction. This field describes the outermost atoms terminating the surface slab or nanoparticle cluster. For example, for a Pt-Pd alloy material, Pt, Pd, or mixed Pt-Pd terminations are possible.
String

Surface (1st Layer) Composition

Chemical composition of the surface layer.
String

Sub-Surface (2nd Layer) Composition

Chemical composition of the sub-surface layer, i.e., the layer between the surface and the bulk.
String

Cell Symmetry

This field describes the relation of the surface slab to a single unit cell, in both size and relative shape. For example, 1x1 indicates a single instance of the unit cell in the periodically repeated directions (typically x and y), whereas 2x2 would indicate four total unit cells in the slab. Other, non-primitive unit cells apply the naming conventions used to describe overlayers in surface science.
String

Nanoparticle Size

For nanoparticle calculations only, how many atoms comprise the nanoparticle cluster? (excluding adsorbate atoms)
Numeric

Methods

Describes the specific density-functional theory methods used to calculate the adsorption energy.

Software

Commercial software package or custom code used for the calculation.
String
Required

Potential

Pseudopotential used to approximate potentials felt by valence electrons as a result of interactions with the nucleus and core electrons, without explicit calculation of core electron states.
String

Basis Set

Basis set used to expand the valence electron wave functions.
String
Required

Fixed Substrate?

Was the slab or nanocluster model completely fixed during adsorbate optimization (TRUE), or was any portion of the model allowed to relax (FALSE)?
Boolean

Spin Polarized?

Were spin-polarized calculations performed? In spin polarized calculations the up and down spins in each electron pair are not restricted. This method is commonly used for materials and species with a non-zero magnetic moment.
Boolean

Zero Point Energy (ZPE) Corrected?

Were the calculated energies corrected using the zero-point energy?
Boolean

Adsorbate and Reference Species: Overview

An adsorption energy is always calculated as the energy of a surface-adsorbed molecule relative to the sum of the energy of a) the "empty" surface and b) an isolated gas-phase molecular reference. The gas-phase reference may be the same molecule as the adsorbate but simply isolated in space, or it may be a different set of molecules having the same overall formula as the adsorbate. It can even be composed of one or more molecules minus another, such as H2 on a Pt surface being referenced to Pt and H2O – 0.5 O2.

Adsorbate

Adsorbate Species

The molecular species adsorbed on the surface. The Adsorbate/Reference Species section below provides more details on the info included for each species.
Species Record
Required

Adsorption Site

Metadata describing the surface site of adsorption and/or binding configuration. Note that this field is not standardized. Users should consult the linked reference for site and configuration naming conventions.
String

Coverage Fraction

Surface coverage of the adsorbate species (in units of monolayer, ML), typically defined as number of adsorbates/number of surface sites. Represented as a fraction string. What consititutes a surface site for this data value is defined in the Coverage Fraction Unit.
String

Coverage Fraction – Numeric

A numeric (decimal) value for the coverage fraction, calculated automatically from the Coverage Fraction string.
Numeric

Coverage Fraction Unit

Defines what constitutes a surface site (e.g., for a Pt(111) site, the coverage fraction would be per Pt).
String

Gas-Phase Reference Set

The reference set is composed of one or more pairs of a species, from the adsorbate/reference species table below, and the coefficient for that species in the balanced adsorption reaction (e.g., 1 for H2O and -0.5 for O2 in Pt + H2O – 0.5 O2 → Pt–H2)

Reference Species

The gas-phase reference species. Each database entry must have at least one. The Adsorbate/Reference Species section below provides more details on the info included for each species.
Species Record
1+ Required

Coefficient

The stoichiometric coefficient for the reference species in the balanced adsorption energy equation. The coefficient is multiplied by the calculated gas-phase energy of the reference species when computing the adsorption energy.
Numeric
1+ Required

Adsorbate/Reference Species

Details on the chemical structure of adsorbate/reference species.

Connectivity SMILES

A version of the SMILES that contains only connectivity information, replacing all multiple bonds with single bonds and adding explicit hydrogen counts, where needed, to preserve connectivity and structural formula. Because the bonding of the adsorbate species is often indeterminate, species having the same Connectivity SMILES are treated as identical in the CPD.
String
Required

Name

The chemical name for the species (common or IUPAC).
String

Adsorption Energy

Computed adsorption energy data.

Adsorption Energy, eV

Calculated adsorption energy in electron volts (eV), denoting the strength of interaction between the adsorbate and substrate. This is the key output of the calculation.
Numeric
Required

Most Stable Site/Configuration?

Of the adsorption sites and configurations studied in the paper for this surface and adsorbate, was this the adsorption site/configuration found to have the lowest energy (i.e., energetically preferred)?
Boolean
Required

Source Metadata

Information on the source of the record.

DOI

Digital object identifer (DOI) for the peer-reviewed publication that the data is reported in.
String
Required

Uploaded By

Name, email, and institutional affiliation for the user who uploaded the data point.
String
Required

Notes

Additional details, if any, that did not fit into other fields.
String