Table 2. Some of the Most Important Properties for Development and Validation of Force Fields and Training of Neural Networks.

property	symbol	sourcea	importance
Energetics
energy relative to atoms	E	QM	for enthalpy of formation
excitation energy	E*	QM	for reaction kinetics in reactive models
conformational energy relative to minimum energy	Econf	QM	for intramolecular potentials
interaction energy in dimers and complexes	Eint	QM	for intermolecular interactions
force vector in nonequilibrium conformations or complexes	F	QM	for both intra- and intermolecular interactions
vibrational frequencies	ν	B	force constants and thermochemistry, dynamics
second virial coefficient	B	X	gas phase intermolecular interactions
Thermochemistry
enthalpy of formation	ΔfH⊖	B	validation of intramolecular interactions using experimental data
gas phase entropy	S⊖	B	for dynamics and free energy calculations
gas phase heat capacity at constant volume	CV	B	for temperature dependence of (mainly) bonded potentials
Electrostatics
partial charges	Q	QM	to model electrostatic interactions
polarizability tensors	α	QM	to model polarization response
electrostatic potentials	VESP	QM	for training electrostatic models
dipole (or higher multipoles)	μ, θ, ...	B	validation of electrostatics
Condensed Phase Properties
lattice enthalpy	ΔHlatt	X	intermolecular interactions
enthalpy of vaporization, sublimation	ΔHvap, ΔHsub	X	intermolecular interactions and phase change
density	ρ	X	intermolecular interactions
solvation free energy	ΔGsolv	X	intermolecular interactions
heat capacities of liquid	CV, CP	X	temperature dependence of enthalpy
enthalpy of mixing	ΔHmix	X	intermolecular interactions
excess molar volume of mixing	ΔVmix	X	intermolecular interactions
melting point, boiling point	Tmelt, Tboil	X	temperature dependence of intermolecular interactions and phase change
surface tension	γ	X	interface polarization and stiffness
dielectric constant	ε	X	balance between dynamics and interaction strength
viscosity and diffusion coefficient	η, D	X	temperature dependence of interaction strength
crystal structure coordinates and lattice parameters	r, a, b, c, α, β, γ	X	atomic radii as well as intermolecular interactions in the solid state

^aThe predominant source of data is indicated by either X (experiment), QM (quantum chemistry), or B (both).