2.1. System

DeePMD-kit takes a system as data structure. A snapshot of a system is called a frame. A system may contain multiple frames with the same atom types and numbers, i.e. the same formula (like H2O). To contains data with different formula, one need to divide data into multiple systems.

A system should contain system properties, input frame properties, and labeled frame properties. The system property contains the following property:

ID	Property	Raw file	Required/Optional	Shape	Description
type	Atom type indexes	type.raw	Required	Natoms	Integers that start with 0
type_map	Atom type names	type_map.raw	Optional	Ntypes	Atom names that map to atom type, which is unnecessart to be contained in the periodic table
nopbc	Non-periodic system	nopbc	Optional	1	If True, this system is non-periodic; otherwise it’s periodic

The input frame properties contains the following property, the first axis of which is the number of frames:

ID	Property	Raw file	Unit	Required/Optional	Shape	Description
coord	Atomic coordinates	coord.raw	Å	Required	Nframes * Natoms * 3
box	Boxes	box.raw	Å	Required if periodic	Nframes * 3 * 3	in the order XX XY XZ YX YY YZ ZX ZY ZZ
fparam	Extra frame parameters	fparam.raw	Any	Optional	Nframes * Any
aparam	Extra atomic parameters	aparam.raw	Any	Optional	Nframes * aparam * Any

The labeled frame properties is listed as follows, all of which will be used for training if and only if the loss function contains such property:

ID	Property	Raw file	Unit	Shape	Description
energy	Frame energies	energy.raw	eV	Nframes
force	Atomic forces	force.raw	eV/Å	Nframes * Natoms * 3
virial	Frame virial	virial.raw	eV	Nframes * 3	in the order XX XY XZ YX YY YZ ZX ZY ZZ
atom_ener	Atomic energies	atom_ener.raw	eV	Nframes * Natoms
atom_pref	Weights of atomic forces	atom_pref.raw	1	Nframes * Natoms
dipole	Frame dipole	dipole.raw	Any	Nframes * 3
atomic_dipole	Atomic dipole	atomic_dipole.raw	Any	Nframes * Natoms * 3
polarizability	Frame polarizability	polarizability.raw	Any	Nframes * 9	in the order XX XY XZ YX YY YZ ZX ZY ZZ
atomic_polarizability	Atomic polarizability	atomic_polarizability.raw	Any	Nframes * Natoms * 9	in the order XX XY XZ YX YY YZ ZX ZY ZZ

In general, we always use the following convention of units:

Property	Unit
Time	ps
Length	Å
Energy	eV
Force	eV/Å
Virial	eV
Pressure	Bar