a	H-activity, -
a 1	Parameter depending on temperature only in Equation (13)
A	Parameter in Equation (6), J mol−1
B	Parameter in Equation (6), J mol−1
b 1	Parameter in Equation (4), mol−1 s−1
E	Interaction energy, J mol−1
F	Helmholtz free energy, J mol−1
f	Fugacity, Pa
f 0	Reference fugacity, Pa
$\overline{G}$	Partial molar Gibbs free energy, J mol−1
$\overline{H}$	Partial molar enthalpy, J mol−1
ħ	Reduced Planck constant, h/2π, J s
$K_{{\mathrm{H}}_2}^{}$	Langmuirian hydrogen dissociation constant, Pa−0.5
Ke	Equilibrium constant, -
n	Number of moles, -
P	Pressure, Pa
r1, r2	Adsorption and dissolution reactions
R	Gas constant, J mol−1 K−1
$\overline{S}$	Partial molar entropy, J mol−1 K−1
U	Internal Energy, J mol−1
T	Temperature, K
V	Volume, m3
Greek Symbols
φ	Fugacity coefficient, -
μ	Chemical potential, J mol−1
μ 0	Chemical potential in the infinite dilution condition, J mol−1
ν	Poisson ratio, -
θ	Surface coverage, -
ξ	H-concentration in the metal lattice, -
ω	H-atoms local oscillations frequency, mol−1 J−1 s−2
ω 0	H-atoms local oscillations frequency at zero H-content, mol−1 J−1 s−2
Subscripts, Superscripts and Abbreviations
ads	Adsorbed phase (on the metal surface)
conf	Configurational
osc	Oscillatory
el	Electronic
g	gas phase
H	Referred to Metal–Hydrogen interactions
HH	Referred to Hydrogen–Hydrogen interactions
ov	Overall
Perm	Permeate side
sol	Solution state (dissolved in the lattice)
Acronyms
BCC	Body-Centered Cubic
BCT	Body-Centered Triclinic
FCC	Face-Centered Cubic